I have no personal opinion as to what really happened, but what you just wrote is backwards.
If democrats mostly voted by mail, then it is democrats who would be able to then also change voting machine results by walking into polling stations.
If you have not yet voted, then you would want to actually vote when you walk into a polling station, not use that vote to try to manipulate the machines instead.
It is those who already voted by mail that could try to manipulate the machines.
When you manipulate the machines, you are not changing your vote, but as many other votes as possible.