Louis explains in several videos on his channel.
They're experimenting with a business model where they ask users to pay for the product if you get value from it. Development isn't free, their time is valuable. In return they'll never harvest and sell your data.
If this experiment is a success it can demonstrate that it's a viable business strategy to not harvest data, which is good for everyone.
Personally, at this point I'm trying out the FUTO keyboard but it's too janky for me to pay for it. Lots of bugs and swipe is not good. I hope it gets better and I'm trying to help the project by submitting bug reports.
Grayjay I've barely used but I see the potential, and if it gets good I'll pay for it. I paid for Signal messenger because it's the same kind of thing.
It's up to you. They're telling you what the price is, it's the honor system if you use it and get value from it.
Translation:
"You know, it'll just buff out bro we build the bridge in front of us as we walk across it bro"