Most owncloud installs assume that you have something like nginx or apache to front-end them, provide security and run the correct script (code file) when requested. These front ends do the https stuff, and would need their config, including the certificate.
Others, more knowledgable, should contribute a definitive answer, but I think its required to run owncloud.
I agree with you.
What I would like to add is that there is no one size fits all answer for setting up a reverse proxy.
There are so many different solutions, lookup traefik, caddy or nginx-proxy where you’re going to have to decide what’s the most suitable for you depending on what you’re trying to achieve.
For example if you’re just trying out ownCloud on you local PC, you will have a hard time using Let’s Encrypt, so perhaps a simple self-signed certificate would be more suitable.
So, finally, I would recommend to pick a proxy solution and move the discussion to their support forum.