The repository for Proton Mail Bridge (which is open source) claims to also host the source for the Import Export app.

Here: https://github.com/ProtonMail/proton-bridge

Briefly looking at the files and code it's hard to tell whether that is still the case, but it's fair to assume Import-Export would reuse most of the machinery behind Bridge.

bridge exposes imap making it easy to download all your data using Thunderbird or another client. I don't know about "export" because imap does what I want and is supported by most providers in the same format.

Does it - legally - matter?

For those who care about OSS and moving to Protonmail from bigcorps email, this _might_ matter, although I think that for the majority of Protonmail users (which doesn't care much about open source), it doesn't matter.

If I'm jumping out a boiling pot I don't care if it is carpet or tile on the other side.