Toilets have their own reservoirs, so the flush wouldn't have any effect on the supply side of the system. And I think that's the point: there's more water stor

Toilets have their own reservoirs, so the flush wouldn't have any effect on the supply side of the system. And I think that's the point: there's more water stored in distributed toilet reservoirs than the city could dump in through a centralized point. Refilling them all at once may exceed the city's ability to supply pressure though. If pressure is provided by pumps, they'll increase their flow rate to maintain pressure until they reach their maximum rate, at which point the pressure will fall. On the sewer side, I could see the flush creating water hammers that damage various parts of the system. I could see them reasoning that they can do the flush and then go ahead and repair any water hammer damage, and that will be easier overall dealing with thousands of blockages.