I don't understand your math. A 12-15% lead isn't mutually exclusive with the trailing candidate having 40% of the vote, and it definitely doesn't require having 65-70% of the vote. Major party candidates have always gotten at least 40% of the vote, even in landslide years, this isn't some new phenomenon (Hoover got 39.7% of the popular vote, and Mondale got 40.6%). I'd love to see how a 12-15% lead requires having 60-70% of the vote, must be some interesting arithmetic to get to that conclusion.
Trump being in the low 40% isn't the part that I find unbelievable, Biden being stuck between 48-51% is what's pathetic. If he were at 55%, he'd have the 12-15% lead I'm talking about, and that wouldn't require him winning over any Trump s. All it would require is him winning the undecided voters + some of the voters who plan to vote 3rd party.
Regarding the EC - not sure if you meant to, but

if you're implying the notion that Biden isn't winning the popular vote by more because he's focused on winning the EC. He's wasting money and resources trying to

make Texas blue

instead of focusing on middle American swing states, and he's parading around GOP endorsements from people in NY, NJ and CA. His campaign seems more focused on running up the score in coastal states than winning middle America.