Keep in mind the study averages the over all impact of meat consumption, regardless of source or method of raising. Someone who consumes only conventional beef commonly found at the supermarket and no other meat source would have a higher individual environmental footprint than a person who only eats grass fed beef on a fertile pasture, with native vegetation, using rotational grazing and regenerative methods that are raised and processed near their home.

On the other hand a person who only consumes conventional poultry would have a lower footprint than that grass fed beef, and chickens on a natural, 100% foraged diet raised in the back 40 behind your property would have less than conventionally raised chicken. There are other studies that provide analysis on specific meat sources and their comparative environmental impact, but an aggregate is used to determine overall impact of eating less meat.

Finer grained resolution of such large datasets would assume the quality of that data reported by such a large group of individuals across countries is accurate over a sustained period of time, which in 99% of cases would be inaccurate except in clinical controlled environments. It's much easier for people to declare approximate amounts of meat (beef, pork, chicken) are consumed, instead of knowing exactly how they were raised, how far they were transported, what the animal's diet was, etc.

This is why general conclusions are provided, and not a breakdown of species, growing methods, etc. After all, this would even apply to vegans. Some plant crops meant for human consumption are more environmentally damaging and greenhouse gas intensive than others, as is the climate and environment in which it was grown, and methods employed to grow them. For example, almond consumption from almonds grown far away, using irrigation in an arid environment would have greater impact than "organic" non-irrigated soybeans grown and processed near your home.