1) Besides being a paleontologist by training, the guy's an expert in horses and is well acquainted with the habits of horses existing today, at least according to the article.
2) He's more likely to know the habits of existing horses than most paleontologists.
3) He's noted that:
Scientists once universally thought the more primitive horses, which lived from about 55 million to 20 million years ago, were primarily leaf-eating browsers, only becoming grass eaters as the prairie grasslands began to spread rapidly across North America during the Miocene Epoch about 20 million years ago, MacFadden said...The reality is not so clear cut, MacFadden said. Actually, during times of transition, some groups of horses actually became mixed feeders, eating both grasses and leafy material, he said...
Although modern horses are primarily grazers, they will feed on fruits and leaves when grass is in short supply, MacFadden said. “Horses are highly adaptable,” he said. “They can exploit different food resources when they have to and are able to withstand a wide range of climates. They live in the tropics. They extend all the way up to the Arctic.”...
The preconceived notion that the horse was once as small as a dog but progressively grew to its present stature now can be proven to be incorrect, MacFadden said...
About 20 million years ago during the Miocene Epoch, horses diversified in size rather than just becoming larger, MacFadden said. While some grew larger, others became smaller or remained the same size, he said.
QED
So you accept this person as an expert on his subject, and you accept his account of horse evolution?