A part of it is by nature, CEO's are public figures, and their name is their brand. Another part of it is, as Mordor says, kn0thing is a handle that just makes its own slogans. Plus, Sam Altman (u/samaltman), Yishan Wong (u/yishan), Ellen Pao (u/ekjp) all have their handles nominally based on their names, though I am assuming Ellen's handle is u/E(llen)K.J.P(ao). What you do see, is the weird middle ground is spez/Steve Huffman, chooter/Victoria Taylor, where they get referred to as both. There is also the case of for the life of me, I can not recall Aaron Swartz' handle, though it appears to have been AaronSw. Alexis Ohanian was always more the type to prefer using an online handle, being a "dwarf" version of a nerd, if you take the Cryptonomicon lexicon. And the fact that personally, i can never remember Ohanian.