overall national turnout was substantially higher in 2020

There is absolutely no way to know this. There were more ballots, yes. But we have no idea how many people actually voted because so many of them were mailed in with zero attempts to validate them. Some places completely turned off even rudimentary signature verification.

Your claim is bullshit because it's an assumption that can never be verified. Even if it happens to be true, the claim itself is bullshit.