Well, sort of-- he deleted the original tweet and added his clarification to that old tweet-thread, rather than tweeting anew. Plus, his original tweet straightforwardly stated as fact that it was a "left-wing activist", whereas the clarification he gave was more mealy-mouthed: "police are saying this now", rather than "this/I was incorrect".
But, yes, it counts for something. But the point of Will Sommer's tweet still stands. The fact remains that Andy Ngo's original (wrong) tweet was retweeted 13,000 times, and his clarification was retweeted 687 times. Sommer's main point was that the correct information to come out afterwards gets a tiny fraction of the attention, and that's how misinformation spreads.