actually, those figures are far tamer than any i'd seen for two stroke engines (non-CO2 emissions).
one i heard was a two stroke engine will emit in one hour, the same pollutants as a modern petrol engine (with catalytic converter fitted) would in 16,000 miles driving.
found this online:
"one jet-ski driven for one 8 hour period emits the same amount of pollution as a car driven for 100,000 miles."
Factsheet on Off-Road Vehicles - Wildlands - Sierra Club