This isn't surprising, what would be surprising is if NPR blocked inbound referers for Twitter and it had no effect on their traffic. All of NPRs articles presumably get posted into Twitter by someone noteworthy, and that tweet is what goes viral (if any), not the one from their main account.

Now that Musk has recently changed the way unfurls work, I would imagine that definitely affected their traffic.

Doctorow has had a chip on his shoulder about Musk for a while now and I would be happy to go out on a limb predicting that any theories he has about Twitter's demise won't age well. The reason Twitter works is because of network effects, and those network effects are a function of Metcalfe's Law, not the presence or absence of a single account on the platform. Being part of the web, Twitter's network effects are intertwined with the web and their social graph. That's about it. The way to know if Twitter is going down in flames is the degree to which Musk breaks the network effects of the web, or if the exodus from the platform turns into some kind of positive feedback loop. Musk seems much more willing to mess with the former than the latter. (Much of the recent changes are about increasing users and time on site)