Yea, it was a ballpark estimate not really a major study.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1466&...

Shows several other HP estimates for horses at up to 1.5 HP per hours and even one at 125lb * 3mph over 6 hours = 1 hp. Also includes a peak from a contest at 14.88 HP.