I previously commented I would write the support, now I actually read the docs first and found out why:
If you want to create an account over tor you can, just not via the clearnet URL, probably due to rate limiting by IP adress. However if you use their Onion Link as specified in this article by their support (https://proton.me/support/tor-setup) it works just fine (as far as I just tested). So great! Because using .onion services is far more secure than accessing clearnet over Tor anyway.
Here the url, verify it with the link in the support article tho: https://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/