I think you can train horses to be aggressive (intact males can often naturally be like this) but whether they will break a formed body of men is another thing. As you say, if the unit has pointy things to jab at/into the horses and doesn’t lose courage, it seems to have usually seen off the horses. It’s the courage to stand fast when you can feel the ground moving as the horses approach that is key.
I presume Loades didn’t have his shieldwall stick sharpened steel pike/spear heads into the horse’s face or chest as it approached? Which is why I normally have doubts about what such re-enactment experiments can teach us. The pain, terror, and in humans, fear of death is missing. What wins out between a horse trained to lash out and kick with steel shod hooves, and a solid wall of sharpened steel points aimed at its eyes, nose and chest and double handed swords and axes lopping its legs off? I’m hoping that no-one is going to do the experiment to find out.