Sounds like the TP is doing the right thing here. Create the challenge and let the real voters prove otherwise. If they succeed, cool. If not, one less fake voter.
I am reminded of an incident that happened in 2004 (might have been 2000) where a local Republican party sent a mailer to everyone on the Voter registration list at their address of record, then challenged all that came back as undeliverable. I also think a few registrars started doing the same thing, possibly the same year as they thought it was a bright idea. Democrats were screaming, but I find it quite likely that anyone whose address of record cannot be found by the post office is illegally voting.