Yes, I understand perfectly well what that word means; 90 is just a number. Why not 100 days or even 80 days. Please get me the studies telling me how one number is better than the next. I'm going to bet you'll be digging a while. So, yes, arbitrarily set at 90 days, so bugs won't linger unfixed.
The ultimate goal of the number is security, not security at all cost. It is not a dogma were no situations can ever alter the number.
The simple matter is that this didn't increase security by releasing the technical details (not just the bug's existence), the stated goal, and Microsoft or Apple releasing a not fully vetted patch had a good chance to disrupt security and stability of existing system. A bigger failure. A failure mind you that Google would not have to bear at all.
So, Google had basically nothing to lose in doing what they did.
Considering the potential massive damage a not fully tested patch could inflict and the fact that the bug wasn't critical (like say the goto bug or the heartbleed bug), a few more days of delays would not have made a big difference.
That's how it works in the real world; not patch as you please, because we don't care what happens elsewhere, world.
If Apple or Microsoft were totally ignoring the fix, you'd possibly have a point, but as things stand, you do not.