This study should be expanded to include wild horses. Also if they could somehow measure "positive" reactions that would be interesting.
Domesticated horses are generally trained through fear; it is not surprising they react to angry faces. So it might also be interesting to conduct this same test on horses trained only with positive reinforcement techniques.
I live in a rural community. I have had many friends and family work with our local light-horse association. I would really appreciate you localizing that comment about trained through fear and reporting it to your local equestrian society. I apologize if you are not from North America and part of a country that allows you do so.
Domesticated horses are generally trained through fear; it is not surprising they react to angry faces. So it might also be interesting to conduct this same test on horses trained only with positive reinforcement techniques.