Symmetry is not guaranteed. If someone creates a deadly pathogen with a long pre-symptomatic period (which we know is possible, since HIV works this way) it could infect essentially everyone before discovery. Yes, powerful AI would likely rapidly speed up the process of responding to the threat after detection, especially in designing countermeasures, but if we don't learn about the threat in time we lose.

There are people today who could create such a pathogen, but not many. Widespread access to powerful AI risks lowering the bar enough that we get overlap between "people who want to kill us all" and "people able to kill us all".

This is not a gotcha argument, this is what I work full time on preventing: https://naobservatory.org The world must be in a position to detect attacks early enough that they won't succeed, and we're not there yet.