Just reading the article you provided and "Feeding high grain diets to cattle unequivocally lowers the formation of CH4 in the rumen." seems to disagree with what you're saying?

Ah, you're right, it is a nuanced question. Perhaps more importantly: as mentioned in the (admittedly large) publication I cited, the rearing time for grassfed cattle is about 3x longer than factory farmed cattle, so the net methane output is considerably larger.