It's not macOS firewall, but Apple's implementation of OpenBSD's pf used in Apple's macOS. Mullvad is clearly pointing at a bug in OpenBSD's "packet filter", mentioning that it's used in macOS.
Mullvad's article lacks proper wording and shits on the wrong target.
You're way off base and I can see you feel quite frustrated by what you perceive as a slight against OpenBSD. I know and have used "pf" in OpenBSD. Not once while reading the article did I think Mullvad were referring to pf as a technology as opposed to the macOS implementation of pf where the bug resides.
Unless there is an equivalent OpenBSD bug, why would it be their issue? Low level components often are patched by Apple to work with Xnu. If the same bug isn’t showing up in OpenBSD, it’s more likely Apple’s integration or a “feature” added by Apple.
Maybe there isn't such a huge intersection between BSD and Mullvad users? It would seem though that Mullvad might try to see if the problem narrows down to the BSD implementation, but that's hard to know if they haven't stated that explicitly.
Mullvad's article lacks proper wording and shits on the wrong target.