As Bob observes, the fakes don't really matter if you're overbooking the venue anyway. I'm sure there are a bunch of flakes at even the well-attended rallies (and under normal circumstances).
But as strategy, it's actually quite brilliant.
If you were living in Tulsa, and you heard that there were a few thousand tickets still available for the rally, you might just decide to go and see what the big deal is.
But if you heard that they sold a million tickets? Even if only 10% showed up, that's a hundred thousand people milling around outside an arena that can fit less than one-fifth that number. No way are you going to get last-minute drop-in attendees with numbers like that floating around in the news.
And then, when 99% of those ticket sales are fakes, they're left with a half-full venue, and egg on their face. The optics of this are terrible, and it forces Trump et al. into even more stupid lies to cover it up.
At one point, he was telling the crowd that they couldn't get into the building because of all the "thugs" who were blocking the way. And everyone in that crowd knows, for a fact, that this is stupid ********. It didn't happen. They saw Trump lying to them in real time about something they saw with their own eyes.
And that had an effect - people were leaving the rally long before Trump was finished blathering. Like football fans in the fourth quarter when the home team is losing 42-0, they decided that beating the traffic home was a better use of their time.
He's not gaining any new voters with this nonsense, and every voter he loses is all to the good.