I skimmed a bit through both profiles, and here are my thoughts.
The stat diff is there (over 4x to be approximate), but the use of a piercing weapon in the form of the knife. Though if that is LSed away, William Shatner would have trouble attacking her. Myers has to be really careful of that huge ass LS, lest he gets into trouble. Although, his regen and immo does allow him to stay in the fight longer, if he doesn't get his bones broken, while would slow him down or put him out of commission. However, I'm not sure whether Haru's go-to-move is to break some part of Myers at first. Overall, Haru has the stat advantage against Michael Myers, tho he isn't a slouch.
The stat amp/reduction is for speed and stamina, and while I'm not sure whether his stamina will be reduced to the point he can't fight her for long, the speed amp + reduction combo kinda probably be just for travel speed (assuming that they are primarily used during races). Even then, it's agreed here that it is mid, so not much might change (especially since Haru's speed is the one getting equalised, which means her travel speed is slower than her opponents).
Enhanced senses are weird. I read up on the horse senses, and while the hearing part is good, Haru's sight stuff is limited by the fact that she's having a human head rather than the normal horse split eye. So she's going to have to rely on her hearing to figure out where Myers is. That is, if Michael Myers can actually stealth in an open field in the daytime. I don't doubt his stealth, but he is no Agent 47.
Skillwise, knowing all sorts of martial arts normally doesn't mean you're automatically super skilled. The problem for Myers is that he doesn't normally dodge attacks, as seen in his fights with Busta Rhymes, so she can land hits on him without any issues. However, he is a glutton for punishment, and can just get back up. Her trump card here is Judo, which sufficiently incorporates her LS with grab moves and stuff.
He does have better intelligence than Haru tho. If he's able to exploit that, like by playing dead from her attacks, there's a good chance that he can get a sneak attack on her. Though the roadblock in question is her hearing, which is this by the article. And that's if she isn't stupid enough to assume the noise comes from anywhere else other than Myers.
A horse’s hearing is much keener than ours. They use their hearing for three primary functions: to detect sounds, to determine the location of the sound, and to provide sensory information that allows the horse to recognize the identity of these sources. Horses can hear low to very high frequency sound, in the range of 14 Hz to 25 kHz (human range = 20 Hz to 20 kHz). Horses’ ears can move 180 degrees using 10 different muscles (vs. 3 for the human ear) and are able to single out a specific area to listen to. This allows the horse to orient itself toward the sounds to be able to determine what is making the noise.
All in all, Haru brings an unexpected trouble that can give Myers a run for his money, and neither really overtake each other too much due to their different advantages.