The DSA didn’t “notice” anything. They’ve known the whole time. She left the DSA to join the Democratic party because the Democratic party was the only way to get elected anywhere.

That’s not how it works though. DSA is legally a 501c4. It parasitize the Democratic ballot line, but any DSA member running on the Dem ballot line wouldn’t have to give up their DSA membership.

What happened here was likely that her dues lapsed without her knowing(a thing that happens relatively often, and it isn’t anyone’s fault), and nobody noticed. National has to submit requests to the relevant staff for that information, so they weren’t checking. And NYC-DSA, who would have been the ones responsible for checking when doing endorsements, just assumed she was a still paying dues, when she wasn’t.