Proton requires an account, which gives them some of your info, while Mullvad does not, giving you an anonymous account number instead.
If Proton really doesn’t log VPN traffic, then it doesn’t really matter. But since Mullvad does not have that same personal info, they would be unable to provide law enforcement or 3rd party data brokers any hard data aside from your IP if they wanted to.
I was wondering why it was written in C++, but the FAQ already beat me to it.
Glad to see they are open to using safer languages. C/C++ was great for its time, but we really need to move on from them.