Google publicly disclosing the bug doesn't only let affected users know. It also lets attackers know how they can exploit the software.
Holding public disclosure over the heads of maintainers if they don't act fast enough is damaging not only to the project, but to end users themselves also. There was no pressing need to publicly disclose this 25 year old bug.
How is having a disclosure policy so that you balance the tradeoffs between informing people and leaving a bug unreported "holding" anything over the heads of the maintainers? They could just file public bug reports from the beginning. There's no requirement that they file non-public reports first, and certainly not everyone who does file a bug report is going to do so privately. If this is such a minuscule bug, then whether it's public or not doesn't matter. And if it's not a minuscule bug, then certainly giving some private period, but then also making a public disclosure is the only responsible thing to do.
Holding public disclosure over the heads of maintainers if they don't act fast enough is damaging not only to the project, but to end users themselves also. There was no pressing need to publicly disclose this 25 year old bug.