The theory is partly a hangover from 2016. It’s true that pollsters undercounted Trump voters in that election. But it’s a huge jump to think it was because SMAGA (Secret Make America Great Again) voters were refusing to state their preference. A postmortem by the polling industry went looking for significant numbers of SMAGAs and came up empty.

There aren't SMAGA voters, but don't anyone get cocky - everyone should vote.