This is not a "concern", this is literally what happens. Even prior to AI vulnerability scanning, release notes and diffs for any normal release or security patch of popular software were analysed and used to develop exploits, which could then be applied against non-upgraded instances. Nowadays, AI is doing this en masse and automatically producing exploits. And, as has been demonstrated, nowadays even the rumour of a major security bug is enough for people to find it, as people will immediately burn huge volumes of tokens looking for it.

Nor can you just keep it quiet and smuggle it out in a regular release as you suggest. For one thing, changes in important software are be linked to commits and have pull requests, reviews, etc. Changes without these will immediately raise suspicion, as either a malicious change snuck into a release, or as a "quiet" fix to a major vulnerability, completely undermining the purpose of keeping it quiet in the first place. Moreover, major vulnerability fixes cannot be "quiet", because the whole point is to get as many people as possible to upgrade as quickly as possible. If PHP (or whoever) push out a release containing a major security fix, but don't make this clear, then many people will be in no rush to upgrade, and will get pwned.