Considering that they had real security people before it went private, and it strongly reflects the owner's demands, it's going to be on the hardcore owner. However, everybody does know it's his by now.

Because if they just said “X” people would assume it was a copy-editing mistake. Truly terrible branding. Remember The Artist Formerly Known as Prince?

I think it's written that way as an aid readers in understanding the topic of the article. X is a tremendously stupid name and it's not guaranteed that people will immediately be able to identify it as the social-network-formerly-known-as-Twitter, so making the connection to Elon Musk helps people put the name in context.