The shape of the cows is exaggerated, but not by much - many breeds of cow are just incredibly rectangular in profile.
https://images.squarespace-cdn.com/content/v1/5475e650e4b0df...
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https://images.squarespace-cdn.com/content/v1/5475e650e4b0df...
Doing the work myself, as you have, I find that many cows are rectangular. The author is unequipped to dispel me of this notion, if it's incorrect, but could have easily if he had addressed it preemptively by clearly stating his actual thesis. (Unrealism, not rectangleness). (And if he is correct)
While it might be possible to take a photo of a cow that turns out looking spherical (due to the lighting or angle), it is surely going to be harder?
Likewise, there is contrast between how highland cows are represented - from the front "the cows look like they’re posing for an album cover.". They do indeed look like a cow metal band could. They dont look rectangular, they look hairy.