You can never be sure that you're the only one in the world that has discovered or will discover a vulnerability, especially if the vulnerability can be found by an LLM. If you keep a vulnerability a secret, then you're leaving open a known opportunity for criminals and spying governments to find a zero day, maybe even a decade from now.
For this one in particular: AFAIK, since the codec is enabled by default, anyone who processes a maliciously crafted .mp4 file with ffmpeg is vulnerable. Being an open-source project, ffmpeg has no obligation to provide me secure software or to patch known vulnerabilities. But publicly disclosing those vulnerabilities means that I can take steps to protect myself (such as disabling this obscure niche codec that I'm literally never going to use), without any pressure on ffmpeg to do any work at all. The fact that ffmpeg commits themselves to fixing known vulnerabilities is commendable, and I appreciate them for that, but they're the ones volunteering to do that -- they don't owe it to anyone. Open-source maintainers always have the right to ignore a bug report; it's not an obligation to do work unless they make it one.
Vulnerability research is itself a form of contribution to open source -- a highly specialized and much more expensive form of contribution than contributing code. FFmpeg has a point that companies should be better about funding and contributing to open-source projects that they rely on, but telling security researchers that their highly valuable contribution is not welcome because it's not enough is absurd, and is itself an example of making ridiculous demands for free work from a volunteer in the open-source community. It sends the message that white-hat security research is not welcome, which is a deterrent to future researchers from ethically finding and disclosing vulnerabilities in the future.
As an FFmpeg user, I am better off in a world where Google disclosed this vulnerability -- regardless of whether they, FFmpeg, or anyone else wrote a patch -- because a vulnerability I know about is less dangerous than one I don't know about.