This just adds to the number of times Trump has been prematurely declared politically dead. When it comes to Trump, Mainstream Media commentators have a terrible record as prognosticators. I think your first instinct was correct: turnout for Biden will be very anemic. Biden's campaign will be very lackluster with few rallies and more than a few gaffes. But his biggest problem is that the last few weeks have given Trump a big issue: Law and Order. He will be able to paint the Democrats as the party that let rioters run rampant and abandoned police stations. Many urban business-owners and suburban moderates, nominally Democrats and not fond of Trump, are fearful of what happens if the Democrats win. A few more riots and you could get a stampede to Trump. Can't happen? Well it did in 1968 and got Nixon elected.
The further Left the Democrats get pulled, the more they risk losing moderates and independents. However, if Biden takes a stand against the new items in the Woke Agenda, for example opposing, as he has, "Defund the Police", the greater the chance he has of riling up and losing his Left wing which was not fond of him to begin with. To the degree the Empty Suit has to take a stand on something, he risks alienating one or the other part of his coalition. Also, if he really flubs badly against Trump in the debates, he could get many to quietly sit this one out.
As for the polls, I believe they are registering dissatisfaction with current events, with Covid-19 and urban unrest, and not necessarily any great enthusiasm for Biden. Their accuracy is questionable and their margins of error greatly understated. They badly mis-predicted the 2016 election in at least a dozen states. Who exactly they are polling is hard to say. There is a fundamental problem that those responding to the polls are not a representative sample, not even for any particular cell of the population.
Possibly what is happening is an increase in virtue-signaling hypocrisy where a voter doesn't want to say to a stranger that they are thinking of voting for Trump. The more ominous sign for Biden is that he has a much smaller percentage of very enthusiastic voters than Trump does.
Finally, there is the wild card. Suppose Democrats get their way and there is a vast increase in mail-in voting. Suppose voter harvesting is allowed in more states. The result is that both sides will cheat a whole lot more. Who will cheat better in enough states is an open question.