This explanation doesn’t make sense mathematically, though. If the ratio of Democrat to Republican voters is fairly consistent amongst groups, increasing the number of people in any given group that votes will result in that same ratio, just with greater numbers. Saying otherwise is like saying you can add 2% milk to 2% milk to eventually get back to whole milk
The only way voting by mail helps Democrats is if the vote by mail crowd has a heavier democratic skew.
Let me show you an example. Let’s use easy ratios for easy numbers, so let’s say people vote Democrat to Republican 3:2.
If you have 100 people vote, you’ll end up with 60 democratic votes, 40 Republican.
If you have 200 people vote, you’ll have 120 Democrat votes, 80 Republican.
Increasing the number of total voters in this scenario will never change the outcome. 400 people? 240 to 160.
The only way getting more votes by mail will help Democrats win an election is if the ratio of Democrat to Republican voters is higher in mail-in voters compared to other population groups.