This is a Biased Sample Fallacy - anecdotal and with confirmation bias.
However, Trump doesn’t have to lose 80% of his supporters to lose. Even even if Biden doesn’t pick up a single vote beyond the 66 million Hillary got and Trump gets 97% of his voters to come out for him again, that margin of just 3% don’t even have to vote for Biden, they just simply don’t vote or pick a third party candidate, and that alone will be enough to swing the results.