While one should always be alert to the possibility of systematic polling errors in either direction, this (alleging that polls are biased by 5-6 points against Trump because of shy Trump voters) is completely insane.

https://www.bloomberg.com/news/articles/2020-09-01/jpmorgan-says-prepare-for-rising-chance-trump-wins-second-term

There's not really much/any evidence of shy Trump voters at all. He only beat his polls by ~2 points in 2016, which can pretty easily be explained by not properly weighting by education + late-deciding voters breaking toward him.

Also worth noting that polls in the midterms showed no bias in either direction. Nor did Trump over-perform his polls in the 2016 primaries. Nor do nationalist parties have any history of outperforming their polls in a large sample of European elections. https://fivethirtyeight.com/features/le-pen-is-in-a-much-deeper-hole-than-trump-ever-was/

Again, one must be aware of the possibility of correlated polling error *in either direction*. I can imagine reasons that polls would underestimate Trump again. Can also imagine some reasons they'd underestimate Biden especially if pollsters are "fighting the last war" from 2016.

But usually, if someone is obsessed with shy Trump voters, it's a sign that they have a sophomoric understanding of the field. It's the sort of thing that makes you sound smart but doesn't really have much evidence behind it once you've done the research.