While flooding is a problem, I've never seen siphoning be a problem on an inverted 2-stroke. When inverted, the spraybar is below the port in the crankshaft, so when a siphon starts, it just pours the contents of the tank out through the spray bar, and then down, out of the carb and on to the ground (or into the cowl). I've actually seen this done. Except for all the fuel on the ground, it was pretty funny. "I swear I filled the tank!". I had a side-mounted engine that was also prone to this if I wasn't careful. It wouldn't flood the engine, but it would empty the tank (half way, in this case) if you got a siphon started and didn't notice and didn't get the engine started.

With an upright engine, the siphon can fill the crankcase rather quickly, but siphons are less common.

But in general, you are correct, it is much easier to flood and hydro-lock an inverted 2-stroke, and that can cause problems.