It was enough water to fill that room in a few minutes, I’d say a few toilets and shower drains would be a negligible effect. Although once the water got a few feet over the bowl, or other drain, I’d stay the fuck away from them because of potential delta-P situations.
I wouldn’t know. There are toilets in the US that use a high pressure flush, but they still have a siphon. It’s there so that the toilet can flush without water pressure from the mains (you can flush a toilet by dumping a bucket of water in it) and it also forms a seal that prevents sewer gas from flowing back up through the toilet. So even if the toilet uses an alternate flushing method, it probably still has a siphon.