By checking for fealty to the false “stolen” narrative at the outset, Trump’s allies aren’t simply weeding out people who disagree, they’re weeding out people who won’t acquiesce to the Trumpian approach to reality. They’re not just getting rid of people who won’t go along on this one subject; they’re getting rid of people who won’t go along in general.
That’s why they’re also trying to manipulate the voting. E.g. making it really difficult to vote, or suddenly removing loads of people.