Genuine question - how is it useful for them to release the server-side code? How do you know for sure that the code they have released is the same as what they

Genuine question - how is it useful for them to release the server-side code? How do you know for sure that the code they have released is the same as what they are running? Having the server code would be useful for self-hosting, but if you're relying on Proton to host the code for you, then you have to have a certain level of trust with them regardless.