You’re collapsing progressives and leftists
and you're collapsing progressives and centrists.
A progressive Democrat winning a red seat inside the party (as in working with and within the party like Rehmet did) is not proof that Democrats should “stop attacking the left.”
I doubt there's much you would consider evidence that the party should stop attacking those to your left like they did with Mamdani.
It’s proof that better Democrats, even progressives in the party, can win and don’t need to resort to attacking their own potential coalition partners in the rest of the party.
I mean, the animosity didn't come out of nowhere. This has been going on for decades, and it's not progressives who betrayed the center.