Good day to you, Miguelángel Federico-Hassan. Like you, I am a user who, according to the Stack Exchange's automated script, "prefers to keep an air of mystery about them." I'm pretty sure that the script is not supposed to be taken as snooty, snobby, or unwelcoming; rather, it amounts to nothing more than a playful (or at least wry) acknowledgment that the person so described has chosen not to fill out the biographical template, as the site would have preferred.
A truly snide or unwelcoming message would say something like "This user has chosen not to promote a friendly community feeling on this site by sharing a little something about himself or herself" or "Apparently, this user prefers not to let it be widely known that he or she has two heads and never matriculated beyond third grade." Those messages I would have found obnoxious.
In my view, it's a point in the Stack Exchange's favor that participants are allowed decline to describe themselves and their background and instead to be (by default) international hominids of mystery. We even get assigned a nice, colorful, recognizable geometrical pattern in place of a photo or illustration of our own choice, if we prefer.
Ultimately, I hope, participants at this site are gauged on the basis of the quality of their contributions (in the form of questions and answers) and not on where they come from or what credentials they may have amassed. Nevertheless, people do like to know more about those with whom they interact over a period of weeks or months or years; and having a sense of the global distribution of English Language & Usage participants is, if nothing else, an excellent curative for parochialism. It's not unreasonable for users of this site to take an interest in such things, nor is it unreasonable to choose not to share details about yourself if you don't want to.
My advice is not to take the Stack Exchange automated script seriously. I doubt that whoever composed it spent more than a minute or two coming up with it, and it shouldn't occupy any more of your time than that either. Participation at EL&U is structured in a way that is actually quite respectful of privacy. If my choosing to exercise that privacy option prompts the automated script to attribute an "air of mystery" to me, it's a negligible price to pay. I know I'm not really mysterious.