You misread my post.
The part of my post about the Hiera Ile actually has nothing to do with the discussion about wether or not horse armor was used by Antigonids. The way I see it this source suggests that the bodyguard cavalry (= Hiera Ile = Sacred squadron) was armed this way. There's no mention of the remaining Antigonid heavy cavalry (except that they were scarce at at that time, due to the lack of horses in the Macedonian mainland).
To come back to the Antidonid companions: I likewise haven't found a source yet that denies the use of armored horses around that time. The way I see it it's more a matter of guessing what could have been the custom. Either their horses wore armor, following the trend of the Seleucids (which was a major, arguably the most powerful Successor state at that time), or they were still riding unarmored horses like the original companions. As I suggested in my first post I'm for a compromise: a lighter form of horse armor, only things like the chest and eventually top of the head protected (so not at all full cataphract armor).
But then again it's up to KLA to decide
Edit: Here are the ones made by Avetis: