It's cool. But it does make you wonder if it's programmers who like BTC. Or, lazy programmers who had to come up with a recovery method and did a copy - paste - edit of something else and poof a known working way of generating something that has been audited every which way and is known to a lot of people.
Does it matter on the type of person for implementing it? As long as its been implemented properly, and securely, it shouldn't matter. As you probably know, a lot of programmers rely on libraries, and basically copy, and paste code to make a functional product. I find lazy has a negative stigma around it, but in terms of programming, and copying code that has already been proven to work, and of course you're allowed to do so, I don't see that as the negative
lazy, but rather efficient.
In any case it is never a good idea to use a bitcoin key for anything else or vice versa. This was just interesting to see how a bitcoin proposal finds its way to other fields that have nothing to do with bitcoin.
However, if you know what you are doing and if they allowed entering your own entropy you could technically use your bitcoin mnemonic to derive a child key at a certain derivation path (eg. m/1853125232/0' :1853125232 is equal to prtn) and use that as a recoverable entropy from your main mnemonic.
This is my takeaway from this. Just because Proton mail uses a similar system to Bitcoin seeds, doesn't mean a user should use the same words as their recovery seed of their wallet. Obviously, this is basic security, but I bet a lot of people aren't going to heed that advice.
Though, I haven't checked whether the words correspond to the words that Bitcoin wallets use, which if they don't is probably a better idea, and was probably purposely implemented that way by the developers to avoid this sort of issue.
