It’s not only the “shy” voter but just like 16 there will be a lot of folks who haven’t voted in a lot of elections showing up for Trump. Polls don’t catch them because they are considered “unlikely” voters if target get polled at all.
They either fell for the fear mongering in 16 or felt Trump was just another politician making promises he had no intention of keeping. 4 years later they know better and will show up.
Yes, Cahaly says that “low propensity voters” (people who have only voted once or twice in 20 years) are the biggest group of new voters he is finding.
It reminds me of those after rally reports that almost always show about 25% of the people at Trump rallies have not voted in 2016 or at all.