The whole security embargo things seems incredibly stupid. OEMs are always too late rolling out security patches. So Google thought, "let's create an embargo of months so that the OEMs have time to integrate the patches". Anyone could see it coming that nothing would change and the OEMs would still wait until the very last moment.
So now everybody is off worse. Not only are OEMs still slow with security updates, while CVEs float around for months among those within the know (or reverse engineering skills) for months.
Patch embargoes are a debate as old as security.
I think the overall source embargo is rational _until_ fixes appear in a released binary build. Otherwise there's an integration/QA/rollout window where a source patch is public while the binary patch is unavailable to anyone, including attackers, which is undesirable. In "full" open source this has always been a time-suck mental gymnastics exercise around hidden mailing lists and obfuscated commit messages (which probably aren't useful in the LLM era anyway). It makes sense for Google to avoid engaging with that given they don't need to; I think it would be fully logical for them to perform source drops gated on the rollout cadence to the first available binary release channel.
I fully agree the slower-than-Pixel "vendor lead time" windows are really detrimental. Once the binary patch is out, the source patch and disclosure is effectively out too; those extra windows just let OEMs continue to be lazy as a matter of policy (which they love to do regardless) while exploits are already available.