I don't know where your "scientist" came from, where they went to school but that person makes very little sense.
If mail in ballots should be even between republican and democrats then explain my state which is 100% mail in ballot. No one can vote in person in my state.
Here are the results for my state for 2020. If the number of democrat and republican votes should have been equal, then my state would have ended up in a tie.
View attachment 573344
Same with Oregon:
View attachment 573347
Finally, trump and republicans spent nearly a year telling his followers to not vote by mail. While the democrats spent that same amount of time telling people to vote by mail.
Of course there will be more democratic votes by mail than republican.