I also found the immediate deletion of this post odd when I first learned of it. However, you can still edit the post and flag for mod attention asking for it to be undeleted once it has been improved. So it seems to me that you do still have a chance to elaborate on your answer.
I did find a post where Kit Z. Fox said
it would be unusual to delete a post without giving the author a
chance to comply with the change in policy.
So it doesn't seem to be the usual course of action. However, this does not say that this would never happen, or should never happen.
I agree that the moderator's comment doesn't suggest editing the answer, and I don't know if this was the moderator's intention. I just see this as a potential course of action. The site mechanics allow you to edit deleted posts, and if you do so to fix the issues identified by the moderator, I don't see any reason why the post should remain deleted.
Why might a moderator have viewed this as plagiarism?
Definitions of plagiarism: it may include more than just omitting citations
Plagiarism is complicated to define. The most obvious case is copy-pasting without a citation. However, many definitions encompass using others' words, even with a citation, when it is not made clear that the wording as well as the idea comes from the source. In other words, copy-pasting specific sequences of words, even with a citation, may be considered plagiarism if there are no quotation marks to indicate that the wording comes from the cited source.
Here are some examples of this definition of plagiarism:
Forms of plagiarism
Verbatim (word for word) quotation without clear acknowledgement
Quotations must always be identified as such by the use of either
quotation marks or indentation, and with full referencing of the
sources cited. It must always be apparent to the reader which parts
are your own independent work and where you have drawn on someone
else’s ideas and language. [bold italics added]
When you paraphrase, your task is to distill the source's ideas in
your own words. It's not enough to change a few words here and there
and leave the rest; instead, you must completely restate the ideas in
the passage in your own words. If your own language is too close to
the original, then you are plagiarizing, even if you do provide a
citation. [bold italics added]
This is also mentioned in the Wikipedia article "Plagiarism" that is linked to from the answer to the Meta SE question Users are calling me a plagiarist. What do I do? that MetaEd mentioned. Wikipedia says:
According to "The Reality and Solution of College Plagiarism" created
by the Health Informatics department of the University of Illinois at
Chicago there are 10 main forms of plagiarism that students commit:
- Providing proper citations, but fails to change the structure and wording of the borrowed ideas enough.
You (I'm talking to the reader in general, not to Josh in particular) may define plagiarism differently. My point here is not to say "this is plagiarism", or to "charge" Josh with plagiarism. What I think these citations show is that it is not unreasonable for a moderator to consider this kind of thing plagiarism. If you think we shouldn't define this kind of thing as plagiarism on this site, you should ask a new Meta question to try to establish a site policy about this.
The answer in question
The Help Center page "How to reference material written by others" says
Plagiarism - posting the work of others with no indication that it is
not your own - is frowned on by our community, and may result in your
answer being down-voted or deleted.
It also says that when quoting external sources, "use their words and ideas to support your own".
Josh's answer uses few words of his own. The answer is nearly all quotation blocks, except for a summary paragraph at the start.
Although the summary paragraph does not use quotation marks, its wording is actually very similar to that of the cited source. In particular, there is one sequence of fourteen words that is identical in the source and the answer. Here I have bolded and italicized the most noticeable identical parts:
The two terms "Baptism" and "Christening" are often used
interchangeably as their connotations mean basically the same thing.
However there are significant literal and historical differences among
which the more important is probably the fact that Baptism is a
sacrament while Christening refers to the ceremony of baptizing a
The source for comparison:
Most people use the words "Baptism" and "Christening" interchangeably.
This is OK in the sense that the connotations mean basically the same
thing. However there are significant literal and historical
differences often forgotten, overlooked, or misconstrued.
Baptism: Baptism is a Greek word. Prior to Christianity, baptism was
the ritual use of water for purification. Christian baptism is defined
as a sacrament marked by the ritual use of water and admitting the
recipient into the Christian community. This is the traditional term
used and is an official sacrament of the Catholic Church. Baptism
practices vary between churches, however it almost always involves the
Trinitarian invocation ("I baptize you in the name of the father, the
son, and the holy spirit"). In some cases recipients are fully
submerged in water, and in other cases it may be poured or sprinkled
over the head. The earliest non-biblical forms of baptism were
referred to in the Didache around 100 AD. This reference speaks to the
baptism of adults rather than children. Around the same time we have
references from others about infant baptism being customary. From the
3rd century, onward, groups of Christians baptized infants as standard
practice (although some families preferred to wait until the child was
Christening: Introduced in the 14th century, Christening is the
ceremony of baptizing and naming a child. It comes from English
culture and isn’t properly defined in the modern day. Many
dictionaries will refer to "baptism".