Nate seems to be implicitly arguing out that that claim would not have been inaccurate if made in 2016 (and further that the claim that may have been implied but was not made that the same faction is positioned for success in the general election should it succeed in the party nominating contests would also.have been untrue if made in 2016.) But that is silly, since it's a claim about how the 2020 landscape is difft than 2016.
Or, maybe Nate is conflating endorsements with predictions rather than advocacy, and trying to argue that the rate of success of the groups endorsements is somehow indicative of the reliability of their descriptive statements.
Or maybe Nate, who aside from running a prediction site is also an activist aligned with the faction being criticized against the one doing the criticism, is not posing a rational argument at all, just trying to provoke an emotional response by posting something that vaguely insinuates that the criticizers are wrong in some way.
Why doesnt Nate reply to Neera Tanden, CAP or Nancy Pelosi with “stats” on medicare for all support which is polling at 72%?
If you're implying I'm pro-Biden, for the record, he isn't even in my top three. I don't read Nate because he supports my beliefs, I read him because I think he has a good-faith interest in accurate predictions (even if it is impossible for a human being to be 100% objective)
Edit: to be fair, if memory serves JD mostly/entirely endorsed primary challenges rather than incumbents. The base rate for primary challengers is pretty low, so ~1/3 is a respectable win rate and I would concede that it's a little disingenuous not to mention that.