While I am in 100% agreement here as Plate Barding is pretty common for a heavy warhorse and be strange to have to waste 2 additional feats to get it. On the other strange hand the write up for the
Cavalier Mount says the following:
The mount is always considered combat trained and begins play with Light Armor Proficiency as a bonus feat.
So by exact wording it is only Light by reading it RAW. Though JJ seems to sort of imply
HERE that the wording was too change when the final version was to come out.
So while I will house rule all three armors for War-Trained it's not exactly what the rules say. Best I can show for additional info.