If they want to be seen as responsible rather than opportunistic, then yeah, they should do a proper coordinated disclosure.
Sure, they have no legal obligation to disclose, but we all also have no legal obligation to buy their services. Blacklisting bad actors like this is the right move to discourage this kind of behavior.
Who cares about how you are seen when you are selling 0day for big bucks? The bad actor makes more money than the 'legitimate' one without breaking any law. Punishing someone who didn't alert distros despite a patch being available encourages the company to simply find flaws and sell them for profit - it pays more to begin with.
>they should do a proper coordinated disclosure.
they did a proper coordinated disclosure, following the industry standard 90+30 process. that is why the exploit dropped 30 days after the patch landed.
the kernel team should have communicated with their downstream about the importance of the patch. that is the kernel security team's responsibility -- and they are much better positioned to do that than crossing your fingers and hoping every reporter will contact every distro every single time there is a vulnerability.
there are very good reasons disclosure works this way, backed by a couple of decades of debate about it.