I agree, especially since the effect – if it exists at all – appears to be so small that it’s hard to replicate. Which means it’s small enough that it would be hard to distinguish magnetic north from geographical north.
And you don’t need fancy theories about cows turning sideways to the sun in the morning but not in the afternoon. The cows just need to always turn their butts to the sun. For cows in the northern hemisphere, this means they will on average be facing north.
I don’t know much about cows, but I can imagine reasons for wanting to face away from the sun: don’t want sun in the eyes, want to have grass lit from behind, instinctively want sun on the hindquarters to keep the butt UV-sterilized, etc.