It's a cash grab. The ad revenue supports the developer. The $20/year is just a better bulk deal for the devs because they arent subject to the whims of people buying ads. The only interesting bit for me is if the $100 app lasts 5 years in its current form or if they'll try to charge more for features down the line.
Just going off the 10+ year history we had with this developer on Reddit, he has operated in good faith thus far and added features to Pro without charging for it, so I don't think he'll do that. But who knows. You're buying today with no future guarantees. At least this time we know there's no central organization that can pull the rug out from under Lemmy.
$20 to get rid of ads forever on an app that you use daily is a fair price. No one gets lynched for subscribing to YouTube premium for $14/mo just to get rid of ads