Yeah, I had the same issue, actually reached out to Zapstore. The team is super friendly and they explained that they don't just need source code which Proton cal does publish but an actual APK file. Proton unfortunately has a reputation of being rather difficult when it comes to some of their open source stuff. It's hard to find different services source code and multiple applications Don't provide for example the APK. I've gone to using Fossify Calendar so I can be completely de-googled. (Available on zapstore)

Missing Proton apps coming soon. On it

Added two more. Indexed now but dont expect speedy updates