Since it isn't mentioned in the article, here is the reference: paper (2014)
In the study it even shows how driving a 10 years car for 6000 miles is rougly two years of saved emission with a meat->vegan switch.
I don't know, changing dietary is obviously good for the health, but these results seems to make pretty useless changes, use the bike and save twice as much.
EDIT: There is a new paper (2023), it is in a reply.
I calculated at one point that if you ride a bike instead of a car but replenish the calories with pure beef, it is better to ride the car. So diet matters.