It's not the problem that he didn't correct every single one of them. Problem is, hid didn't even try to correct a single one of them a single one time. See the difference?
This may make EM look a bit sleazy (which I assume was ME's purpose), but it's not unlawful to be sleazy. There's simply nothing in the law that requires EM to proactively correct someone (particularly the press) in this manner. In order for it to be actionable defamation, EM has to make knowingly untrue statements, which even under the most liberal reading of ME's Complaint, EM didn't do.
For clarification, let me lay out the legal theory that ME is proposing and why he has such an uphill battle. ME needs to prove:
1. EM made knowingly false statements about ME
2. Those false statements (that EM was "a" or "the" founder of Tesla, which means by implication that ME was not) cause damage to ME.
3. Those damages to ME are quantifiable in some manner.
Again, I don't see anywhere in the Complaint, which is ME's best shot at his case, where he alleges EM made false statements, only that he failed to correct others. BUT, even assuming that were true, it's a bit of a stretch to say that EM calling himself the Founder of TM instead of ME causes ME damage in some manner. AND, even if he somehow proves that, he has to show what the cost to him is of being "defamed" in this manner. Even if ME proved points 1 and 2 somehow, he'd likely get $1 in actual damages because it will be impossible to prove what he's lost through the defamatory statements.
As for where this case is going -- it's not going to be that exciting. If I were defending EM (and again, I don't know any facts beyond what I've read in the Complaint), I'd file a Motion to Dismiss the whole case. Even assuming all of the facts in the Complaint are true, some or all of the allegations are simply not actionable under the law. Although EM probably wouldn't win on the whole Motion, he could get some of the causes of action dismissed. I would also try to get rid of TM as a defendant -- I don't see where in the Complaint the Company can be held liable for anything. I assume ME just added them to the Complaint to make it jucier.
I don't know ME's financial situation, or how he's paying his lawyers (I doubt they'd take this on contingency, but you never know), but I'd feel pretty good if I were EM -- TM is likely to pay for EM's defense, and they've got to have deeper pockets than ME. Assuming they can poke holes in whatever causes of action remain, they'll likely be able to avoid trial and get a judge to dismiss the case after discovery. What this all means is that there's very little likelihood there will be a trial, although stranger things have happened.