Musk is saying, you haven't demonstrated your numbers are accurate. But having signed the merger agreement and waived due diligence, I think he needs to demonstrate that they are _not_ accurate. Complaining Twitter rate-limited his API access (which would be very foolish on their part) or that their bot measurement standards are "arbitrary" doesn't seem to get there.
The letter complains that Musk asked for data allowing him to "independently verify Twitter's representations regarding the number of mDAU" -- but where does the agreement provide for that verification as a condition of the deal? Maybe Twitter didn't provide sufficient information for Musk to "independently verify" that number -- that means nothing without showing Twitter had a duty to provide that information, or that Musk had a right to his own verification. (Any lawyer agreeing to the other side's verification of such a thing probably isn't a very good lawyer.)
The last paragraph, complaining about firings and hiring freezes and departures, seems positively desperate. Who seriously cares about this stuff?
"I relied on your numbers but now they seem soft" is different than "I relied on your numbers but now they are clearly wrong".
I have enormous respect for his engineering and business skills, he's accomplished some remarkable things. But he doesn't seem so great as a dealmaker. Perhaps he's badly advised, but hey, he chose his advisors.
I would think Twitter will sue for specific performance, and they probably have a case. And Musk is liable for their full market cap, and any trial would depend on a legal team that has a hard time writing a clear letter. I think Musk is in trouble here.