Do people think the club will pull out all the stops to entice people to the Watford game? Slash prices, bring a mate, kids for a quid, family fun day, tons of free tickets to local schools etc?
If they suspect only a few hundred will actually boycott, they might be able to plug that gap with a big one-off push/giveaway, so that they can then point and say there was no drop in attendance, no boycott, etc. That is something I would do, if I was in that position.
Let's say real home attendance circa 6k, but 500-1k boycott. Imagine they could claw most of those back in other sales if they offered hundreds of free tickets and had a huge one-off ticket deal.