Scott Alexander has told many stories about why he is pseudonymous. In 2013, when he was applying for medical residency, he told a simple story which begins with an interviewer saying he would have to take his LiveJournal down if he were accepted:
I originally deliberately linked this blog and some of my other writings to my real name in order to bury Google evidence of certain stuff I did online when I was a dumb teenager, but that mission is pretty thorougly (sic) accomplished and now I agree with my interviewer that even if I don't get hired (cue serious of elaborate ritual hand gestures to ward off unthinkable misfortune) it is to my benefit to switch back to my normal totally un-Google-able pseudonym of Scott Alexander (well, actually just my first and middle name).
FYI, this is why its a red flag that someone has public profiles on dozens of sites. Its a standard technique to bury negative posts in Google results, PR firms will write the profiles and get you a few press releases and talks to help. I hope this is the last thing I have to say about this particular creep.