That doesn't follow. The primary reason to be firm is to ensure that vendors take future issues seriously. Belief that the vendor is serious about a single issue removes only a tiny fraction of the motivation to be firm on deadlines.
1) The issue is so obscure that nobody else in the world will ever discover it, so not disclosing it to anyone but the vendor is the right choice.
2) The issue has been discovered by someone with malicious intent, and every second that you hide the details from the users, they're at risk.
You can't know which case applies, which is why policies about disclosure are useful. If a vendor is informed of a security hole, and they immediately fix it, great, users are saved. If a vendor is informed of a security hole, and they do nothing... eventually users will have to mitigate the risk in their own way (which is usually "stop using the flawed product"). A disclosure deadline strikes a balance; in many cases it's pretty likely that no evildoers have independently discovered the flaw, but would be able to exploit it if they knew the details. So giving the vendor a bit of time to fix the issue is the best solution. But given infinite time, all bugs will be discovered and exploited, so the longer you wait to fix or mitigate, the more risk you take on. Therefore, I think Google's policy strikes a very reasonable balance between protecting through patching and protecting by telling users to use something else.
With that in mind, I have no real qualms with people that disclose flaws immediately (letting users be aware of their risk), or vendors that fix an obscure bug that's not being exploited slowly. In the end, if users want to be free from all risk, they should be finding and mitigating these issues themselves... anything you get for free out of someone else's goodwill is a benefit.
The onus is on MS to show more time is warranted, and so far I’m only seeing evidence to the contrary.
The motivation of the project is supposedly to protect Google's users. Being firm on disclosure deadlines helps ensure that vendors take the issue seriously. Did they have any indication that Microsoft wasn't taking this seriously? If not, then it sounds like their true motivation is elsewhere.