Stuxnet misconfigured industrial equipment that was already set up to run, and all it did was break that equipment. This scenario sets a much, much higher bar.

I guarantee there's misconfigured chemical analysis equipment out there exposed to the internet.

I don't think the grandparent was implying the AI would be controlling robot arms to mix things directly (or at least I didn't interpret as such), but it could very well sit in the network until it notices two dangerous compounds in the same machine, and trigger a breakage that causes a harmful mixture. Break a vial containing a virus, then break two more that cause an emergency evac and maybe that's enough to get something out there.