Eluc on Nostr: Yes I agree, but as a long time Proton user I know more or less how their system ...

Yes I agree, but as a long time Proton user I know more or less how their system works. My guess is that they store the seed in your account encrypted locally the same way that they encrypt the files locally when you send them to drive. Then you can recover using you account password to download and decrypt the seed.
The weak point is the optional recovery email you might have provided, if they allow to recover the seed from recovery, not sure.

That's definitely not the same trust level and security than Sparrow with a hardware wallet or signer but if it's implemented properly that's a good way to start in Bitcoin for any Proton user.