The group doesn't claim to speak for the part’s electorate. It claims to speak for a particular ideological faction, toward which it also claims the party electorate has moved since the last election, so that a candidate representing the faction that was uncontroversially still dominant in 2016 is now out of line with the dominant view in the party.

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.