They may block IP addresses associated with consumer ISPs. Assuming that’s the case, I would guess you’re seeing that as an HSTS/TLS error because their network is trying to trick your browser into redirecting to/displaying an error page hosted by some part of their network.
The repo owner claims to have permission from contributors to relicense (and rewrote some other parts where they couldn’t get permission?): https://github.com/stenzek/duckstation/pull/3295#issuecomment-2348988362
I don’t really understand the rest of that comment though…