I agree with your tautology,
It is not a tautology at all. Ask Nader supporters or anyone who voted for Bernie in a primary.
For instance, you could define better candidate in many ways. For instance, Fetterman is up in 2028. Let's hypothesize for a moment that Fetterman is 100% likely to be reelected. In that case, we'd be sure to win, but then we'd have a Fetterman. Let's suppose also there's a primary challenger cut from an Elizabeth Warren mold. She would be 75% likely to win, but if she does, we'd have another Warren.
It is by no means obvious which of these candidates is "better." In fact, I'd call this "the progressive's dilemma" (without trying to invoke any game theory analogies, as it is not a game theory problem). You can play it safe, take the guaranteed seat and then beat your head against the Fetterman wall . . . or you can take a chance, possibly lose the seat but the upside would be considerably higher.
I am saying that in Maine this year, I want the candidate with the highest % chance of winning the general. No tautology. Now, if the progressive IS the guy with the highest chance, then the dilemma disappears. That's Christmas in politics. It sometimes does happen (2008 presidential election, for instance) so it's not a distant dream.
In general, progressives hate this dilemma -- based on my ample experience. I mean, they should hate it, as it sucks. But the solution to the dilemma is not to erase it with fantastical claims about winning elections by motivating the base. That's just resolution of cognitive dissonance. I'm not saying you're doing that here, but I have come to be quite skeptical over time of the refrain that the progressive candidate is also the one most likely to win. As I said, that happens sometimes, but considerably less often than the reverse. So when I hear "progressive is the one most likely to win," I am suspicious and require convincing.