Your logic is spot on.
But... 4 years ago Nate Silver showed you could forecast a chance of victory for Trump if he was within 2 to 3 percent of Hillary in the national vote. Because she had excess voters in CA and NY.
I presume the same situation still applies. Biden has a "wasted" 1 to 3 percent of the vote due to excess voters in blue states.
So the way the jem algo sees it.... Biden needs to be up by 5 to 7 percent to be outside the margin in an honest poll to be in the lead outside the margin for error.
When you look at polls with an honest turnout model... (1 to 5 more democrats and a proper sampling of independents) ... the race is inside the margin for error... or too close to call.
Hence at the moment it is as you said... its coming down to few swing states...
Even more specifically the outcome probably comes down to "cross over" voters and swing voters in the swing states.
First off kudos to you for making sense in most of this.
Second, sampling should be randomized then weighted, not a predetermined weight prior to sampling. That’s basic statistical mathematics. Saying I want 45 democrats 45 republicans and 10 independents for this poll would ensure inaccuracy.
Broadly speaking, this race will tighten and tighten into Election Day.