My theory is that announcing that this will apply for new accounts and that they are now testing it in New Zealand and Philippines he hopes that people that don't have Twitter account now will create one.

Isn't he the sole shareholder now?

This always sounds nice, except that he could just fire everyone, turn off the servers and close the offices. That would kill Twitter instantaneously. No need to bleed money. Musk genuinely believes what he says and thinks that what he does is the correct thing to do, completely unironically and without any shred of self-awareness. He is just not a very clever man. He did similar hi-jinks when he was the CEO of PayPal, except there were other powerful people there smart enough to fire him so he didn't destroy the company. Same with Tesla and SpaceX, there are other adults in the room that can put him on his place or distract him with shiny toys so he doesn't break anything important. But with Twitter, there is no safety net, this is pure unadulterated, unfiltered, unhinged Muskrat brain juice.