I don't have Proton so I can't check, but I would be very surprised if they didn't base their editor on existing LibreOffice, Collabora, or ONLYOFFICE code. There's a good chance they just improved the UI a bit and added a layer for their encryption to work. A spreadsheet tool and a presentation tool shouldn't take too long after that, I'd hope.
I've seen this stuff implemented in tools like Nextcloud, so I'm a bit surprised this was still missing from Proton Drive to be honest.
