I am presuming at this point that no one is "the President elect" in the eyes of the Constitution until the electoral college votes in December.
In the 20th amendment, the beginning of the sentence is :
If, at the time fixed for the beginning of the term of the President, the President elect shall have died...
So, that means January 20th, according to section 1 of the same amendment. At that time, not only have the electors already voted since at least a month, but Congress supposedly has certified the election of the dead guy and his running mate 14 days prior. That's the time when someone is or isn't president-elect in regards to this amendment.
That's for your first sentence. Now for the rest, and taking your example of the 2020 election, if Biden had died between November and December, the options available to the Electors in each state are different. Some will be able to vote for someone else than Biden for president, possibly someone named by the DCCC, but many Electors in different states will not have that option.
The way I see it, there's two options. Either there are no (or almost no) faithless Electors, then the Biden/Harris ticket gets certified on January 6th (what could possibly go wrong 👀), then pursuant to the 20th amendment Harris gets sworn in as the President on January 20th.
Or, a lot of the Electors who can legally be faithless, get faithless, then the democratic electoral votes are split, no ticket gets to 270, which triggers a contingent election by the House of Reps (and the Senate selects the VP). Who they elect is anyone's guess.
So, if you allow me a bit of presumption as to what you meant, you seemed to imply that Electors voting for a dead guy would be absurd, but in fact it seems like it would be the safest scenario for their party.