Problem is “why now”, but also comparison to username field is not really valid - username is actually useful, what’s the point of birth date apart from age verification? You need some kind of username to converse to fellow engineers, birth date is completely unnecessary for that

One argument I heard about him throughout the years is that he is just afraid of retaliation so he dogwhistles instead but in this day and age nobody who is actually a nazi or conspiracist hiding it anymore and they are not getting any repercussions…

I wouldn’t assume that he supports any of the book content just from having it on the bookshelf. You are not a nazi for having Mein Kampf on the bookshelf (worst example I can think of and it would be uncomfortable at most). If all you have on bookshelf is nazi/conspiracy propaganda though then it’s another topic