The grass eaten by the cows would produce exactly as much methane if it was just left to rot. It's not as if cows somehow produce methane out of grass that would otherwise just....turn into dust?
None of those studies take this into account, they all just measure what comes into a cow and what comes out, but no one takes into account the amount of methane produced from the same feed even without cows involved.
https://www.cbc.ca/radio/quirks/mar-2-2019-the-goodness-para...