I see your point, but I think that might label many uses of wireguard in tailscale "not a VPN" because they use imaginary network devices that only exist inside the tailscale process. Saying that would feel very wrong. On the other hand if process internals can be the deciding factor, then optimizing the code one way or the other could change whether a system is "VPN" or "not a VPN" even though it looks exactly the same from the outside. That doesn't feel great either.

And do we even know if Opera uses internal network addresses for its "VPN"?

I think I'm willing to say that routing all internet traffic from a program through a tunnel can be called either a VPN or a proxy.