The problem is you’re looking at one aspect of a biocycle, a slice, you have to look at the entire life cycle.
Despite its title it does not take into account the entire biocycle, it does not do a carbon cycle assessment either. which the authors implicitly acknowledge "there is no additional warming under a constant level of CH4 emissions"... i.e. they recognize a stable ruminant population does not result in infinite warming.
Also Why i have to give you a source
Because the title of the post is "Eating beef is a great way to prove that you don't care about the environment". So eating natural beef on natural pastures eating natural food doesn't demonstrate a lack of care for the environment.