Like many papers studying this, the authors cite Poore-Nemecek 2018, a flawed study.
One of its biggest flaws is that it calculates using an outdated "global average" which isn't applicable at feedlot level, and especially not to different regional farming methods. The authors use GWP-100, which overstates the effect of CH4, since methane is short lived. They underestimate the carbon benefits of avoiding land use changes by more than 10x (corrected in 2019), and also assume an instantaneous switch to a completely plant based diet, a completely unrealistic assumption. I could go on, but you get the idea.
I encourage you to continue reading the literature, especially the sources that they cite. You will find that many papers cite Poore Nemecek.