Can you provide data on motorcycle engines? I do believe this to be true for 2-strokes and older motorcycles, but for modern Euro-4/5 regulated street motorcycles I very much doubt it.

Carbon emission has an almost direct relationship to the amount of fuel burned, so assuming single occupancy, even older motorcycles do fair better than most cars in that respect. Particulate and NOX emission, not so much.

It's mostly for bikes before Euro4/5. Obviously, new bikes are one thing but most bikes in the USA are offenders of pollution.

A statistic for CA was motorcycles make up 1% of my miles driven but 10% of smog pollutants.

Bikes didn't require catalysts for a long time. And they often used less-precise fueling systems, like carburetors. But that has changed or is changing in a lot of places more recently.

Practically this incorrect. When a car is in single or double occupancy (which is majority of cases) it will pollute more than a motorcycle one. Yes, a filled bus or car might be more efficient, but the average utilization on an auto is very low since most people drive them alone.

You might be talking CO2 emissions where a car emits more than a bike.

Pollution is a different matter though. Even the big 4-stroke tourers have worse emission control than cars; the 2-stroke motorbikes are exponentially worse in SOx/NOx/CO/particulate emissions (which hurt your lungs rather than the climate).