DY
I think the issue is the metzius MAY have changed. The gemara assumes rov cows are kosher. I don’t know remember the technical details but R’ shechter’s holds this to no longer be true and that rov cows are treifa. Therfore every cow would be assumed to be a treifa unless checked, thus making all milk treif unless came from a cow that was checked and found to be kasher.