They do, they keyboard's source is available from their gitlab and you're allowed to copy, modify, and redistribute it, as long as you don't remove the section in the app that lets people pay them and don't sell your derivative works. Sort of a CC-BY-SA-NC deal. Seems fair enough to me — not ideally FOSS, but something I can live with. There are few things I use anymore that aren't totally FOSS but I do compromise sometimes if all of quality, ease of use, security, and privacy gain and the loss in perfect freedom is small enough.