The situation with e.g. BlueHammer is fundamentally different: there, the only party that could act on it (Microsoft) ignored them. In this case, the parties that could act on it weren't notified at all.

I'm also not proposing delaying the disclosure to the general public at all. They already waited 30 days with that, that's fine. Just look a bit further than your checklist of only contacting upstream, and send a mail to the distributions if they haven't picked it up a week or two before.

Downstream vulnerability disclosure is a negotiation between the downstreams and the upstreams. It is not the job of a vulnerability researcher to map this out perfectly (or at all).