Bug reports that apply only to Google's services or which surface only because of them are bugs Google needs to fix. They can and do submit bug reports all they want. Nobody is obligated to fix them.
The other part of this is, of course, disclosure. Google's disclosure of these bugs discredits ffmpeg developers and puts the blame on them if they fail to fix the vulnerabilities. They can acknowledge the project as being a volunteer, hobby project created by others if they want, and they can treat it like that. But if they're doing that, they should not be putting responsibilities on them.
If Google wants to use ffmpeg, they can. But a bug in ffmpeg that affects Google's services is a bug in Google's service. It is not the responsibility of unpaid volunteers to maintain their services for them.