That left hand arpeggio is very strange… it sounds like a little glitch in the piece it's so fast! On top of it, the right hand has a pretty nasty stretch, playing that theme in an octave now.
I wouldn't get too into the nitty gritty of overanalyzing a left hand passage that lasts less than 2 seconds, but it is likely something along the lines of an inverted G7, or maybe (considering the right hand) some weird whole tone thing…. either way, it fits in his harmonic context, and it's effective haha.
If you played Ondine haha, then you've seen PLENTY of polyrhythmic nastiness. Think of the climax in Ondine… like 7/6, 9/7, 5/8, whatever you want, he probably threw it in there!!
I probably got about 4 pages into Scarbo before throwing in the towel (was not part of the rep I was learning… I just had 2 hours to kill and wanted to try something daring lol). Those arpeggios won't get you… it's making them actually sound GOOD that'll get you. Even at half tempo, it's so hard keeping your hands up to each other, and you have to get this demonic skittering theme to cut through on top of it. It's a completely wacko piece.
Try the part about 20 seconds before the [first] BIG climactic chord section all the way into it in Scarbo… now THAT is some absurd writing (in a good way…)