They spent an hour or two deleting tweets on Elon Musk's account, with new tweets appearing soon after. So it seemed like they were aware of his account being compromised but did not immediately [successfully] lock his account.
It’s possible they didn’t understand the scope of the issue for a good amount of time. Elon’s account was the first to drop and was famous in the past for being faked for crypto scams. It’s entirely possible that they assumed it was a single account hijack and avoided notifying the correct people until it was too late. They might not have realized that the account info was changed as well until it was too late.
This must be some new meaning of the word 'immediately' that I wasn't previously aware of. It took them quite a while to get these accounts locked.