He also said, "funding secured at $420 a share", which wasn't true.

^ Jefro118 is correct. This is the a significant phrase in financial and regulatory circles.

How about on Twitter?

It’s irrelevant where it is said so long as it is said publicly.

Edit: This is the same reason Elon is now being sued for "Twibel". You cannot say state lies publicly without legal repercussions. "Funding secured" is classified as material non-public information.

I used to work in finance and regulatory compliance.

I'm guessing there is a HUGE difference between, say, Twitter, and a regulatory filing.

Tesla regulatory filings mention to his twitter account as an official source of news.

There’s no requirement that he lie that way in a regulatory filing, previous posters are trying to explain to you that it’s enough to make the claim publically. Lying in a filing would be a different crime.

You would be wrong, but the bigger issue will be claims of stock manipulation rather than Reg FD violation.

Either the Twitter account is a financial venue covered by the regulations at issue, or making the announcement there was illegal independent of its truth.

So, either way...