I didn’t know this either! Just seen their site, they have several packages: https://www.zrythm.org/en_GB/download.html
I think it is fair enough that they need to make money somehow but looks like they have only made £155 in November which is a shame. I have been a low-level donor for Ardour for 12 years now, and you can see they make something like £4000. I will check out Zrhythm properly as they obviously put a lot of effort in.
That is really a shame! Owh this £155 in November is that only donations? Maybe they also earn via Purchases?