yeeeah, I don't work in suppression systems but have worked with fire for years and now work event production in venues like this. there are other types of fire suppression that aren't classic sprinklers and aren't nearly as visible/noticeable. lack of sprinklers doesn't at all mean a total lack of fire suppression. and I'm not sure how they'd know the building's ventilation situation unless they had access to some the areas where I usually work or have seen a blueprint of the building, which should be available to the public I'd think? but simply going to a show isn't gonna show you what's upstairs, in fact that stuff is generally intended to be hidden or difficult to see for the same reasons we wear black backstage.

if you kept the vent system running the whole time, you're a) burning money on power and b) no point in using fog effects which are ridiculously omnipresent in almost any production these days.