Of course. And neither is Biden's. I see it more as a sampling-type poll. The fair presumption is that the general-only voters will vote in roughly the same ratios as the primary voters. That's a big assumption, of course, but I think it is fair. It may be close, but I don't see any significant risk that Indiana will go blue in November.
Interesting theory. But FiveThirtyEight blog says voter turnout in primaries means nothing for general elections.