Proton's statement from the linked article

"We are aware of the Spanish terrorism case involving alleged threats to the King of Spain, but as a general rule, we do not comment on specific cases. Proton has minimal user information, as illustrated by the fact that in this case, data obtained from Apple was used to identify the terrorism suspect. Proton provides privacy by default and not anonymity by default because anonymity requires certain user actions to ensure proper OPSEC, such as not adding your Apple account as an optional recovery method."

So to be perfectly clear, setting up Wireguard is about bridging two LANs (or devices) to make them virtually appear as if they belong on the same network. For every client that connects they would need to be issued a key and every device would have to be set up. But all the traffic between the two "LANs" would be encrypted and secure.

But I don't think WireGuard is what you're looking for, because this would require setting up all these other people with WireGuard as well. Or doing a more complex setup where you use a VPS and WireGuard and have that serve an exit point instead of your home connection. Or any other number of more complex setups that would work but require a lot more effort... and it sounds like you were just looking for basic port forwarding.

Mullvad took that feature away a couple of years ago (presumably to combat CSAM dissemination). So if you were hoping to just have a secure path for someone to connect to your media server routed through Mullvad, I don't believe that's possible anymore.