I think it’s building on his previous statement that there’s this giant faucet that sends all the water from California into the Pacific:

https://www.c-span.org/clip/campaign-2024/user-clip-the-giant-faucet/5132479

No, they’re piped into the municipal water supply. But if there’s a broad forest fire and a drought the municipal system can lack the available water to pump to the hydrants in the quantities needed for fighting fires. The tap can run dry (and it very recently did). In such a case it’s not untenable to say there’s no water in the hydrants. It’s a bit of an awkward word choice (“no water for the hydrants” or “insufficient water pressure for the hydrants” would have been better), but IMHO there’s better things to criticize him over.