1. Twitter decides to go account only for monetization reasons, and implements this feature with some bugs.
2. People start complaining about the new Twitter policy.
3. Availability issues start being observed, caused by the bugs in the implementation of 1 - possibly a self-DDoS.
4. Elon, in response to 2, lies about 1 being a temporary response to an external pre-existing DDoS attack. People start associating 3 with the claimed DDoS attack.
5. In a hasty attempt to fix 3, whose exact cause they have not yet determined, Twitter starts implementing stringent view quotas. Since 3 was not caused by an external DDoS attack, this actually only makes the problem worse.
I'm not claiming this is definitely what happened, just that it is a plausible time-line of events. The one Elon presented is also plausible, of course.
The best question that could help us distinguish these two cases from the outside is whether Twitter's availability issues started being observed before or after the authentication policy change. I would say that Elon's claim is far less likely to be true if the noticeable availability issues only appeared after the policy change. Conversely, my version of events (which is more or less the same as TFA's, I think) is far less believable if the availability issues happened before the Auth change.
I will note that I didn't use Twitter at all in the last few days and thus don't know which is the case. On HN, I definitely saw the Auth policy change story at least a day before seeing significant complaints about other availability issues, though.