It's also likely this works in both directions - if you support Biden, I bet you don't have a yard sign for it if you live in Mississippi.
By construction, the effect is strongest the more you're not in the majority, which also means your unspoken support is more likely to not matter on the actual outcomes of the election.
Newsflash, they aren't, so your hypothesis must be they're too shy to admit their Trump support to a machine?
Everybody is just fighting the last war where Trump suddenly won for a couple of reasons. So, the Dems are scared they are missing something, and the Reps are going "Haha, who cares about modelling".
Will, the forecasts be perfect? Nope. But is the margin rather large, but not unsurmountable? Yes it is. Are the mistakes from last time repaired? Yes, they take care of uneducated whites. Is there evidence Trump has found a new source of voter support? I haven't seen it.
If anything, Trump supporters have been extremely vocal about who they were supporting, to the extent that they frequently violate social norms and try to takeover events and gatherings to make their political affiliations known, like this week with Among Us.