It's absolutely true that units train to the experience level of remnants. It's obviously easy to test, just grind a unit down ('Charge' Cavalry seem to work best because you can charge them repeatedly to get kills, and leave them in melee to get them killed) and retrain it. Not really an exploit, just an odd decision on the developer's part IMHO. Don't abuse it and it won't change your game.

Ramses II