I think this is the heart of the issue and it boils off all of the unimportant details.
If it's a real, serious issue, you want to know about it and you want to fix it. Regardless of who reports it.
If it's a real, but unimportant issue, you probably at least want to track it, but aren't worried about disclosure. Regardless of who reports it.
If it's invalid, or AI slop, you probably just want to close/ignore it. Regardless of who reports it.
It seems entirely irrelevant who is reporting these issues. As a software project, ultimately you make the judgment call about what bugs you fix and what ones you don't.
[0] More or less. It seems the actual language is shied from. Is there a meaningful difference?
In no reasonable reading of the situation can I see how anything Google has done here has made things worse:
1) Before hand, the bug existed, but was either known by no one, or known only by people exploiting it. The maintainers weren't actively looking at or for this particular bug and so it may have continue to go undiscovered for another 20 years.
2) Then Google was the only one that knew about it (modulo exploiters) and were the only people that could take any steps to protect themselves. The maintainers still don't know so everyone else would remain unprotected until they discover it independently.
3) Now everyone knows about the issue, and are now informed to take whatever actions they deem appropriate to protect themselves. The maintainers know and can choose (or not) to patch the issue, remove the codec or any number of other steps including deciding it's too low priority in their list of todos and advising concerned people to disable/compile it out if they are worried.
#3 is objectively the better situation for everyone except people who would exploit the issue. Would it be even better if Google made a patch and submitted that too? Sure it would. But that doesn't make what they have done worthless or harmful. And more than that, there's nothing that says they can't or won't do that. Submitting a bug report and submitting a fix don't need to happen at the same time.
It's hard enough convincing corporations to spend any resources at all on contributing to upstream. Dragging them through the mud for not submitting patches in addition to any bug reports they file is in my estimation less likely to get you more patches, and more likely to just get you less resources spent on looking for bugs in the first place.