I have read that cyclists who only eat beef generate more CO2 per mile than a hybrid car, though that presumably that ignores all the carbon cost of building cars and car infrastructure. Bikes are still better in pretty much every way.
Do cyclists have drastically different diets than car-drivers? In my experience there is more overlap between cyclists and vegetarians than carnivores.
Even if this is true (I think it's absurd because drivers eat meat too), it doesn't account for the fact that bicycles are slower than cars, so people who commute by bike will travel (much) fewer miles per day than the average car commuter.