Of course it is.
I'm not saying that Argentina should be forced to label their A side as their full test one. My point is that Tier 2 nations should also be able to use official A teams whenever they send an understrength team to a competition like this, for whatever reason. This match should have been considered Brazil vs USA A, with no ranking loss.
Also, I doubt it was the best USA team available. Surely they could have called at least a few of their overseas professional players. They chose not to do it because Brazil was probably considered a weak opponent and they thought they could win with their A team anyway.
I think it's only fair that Tier 2 sides should be allowed to send A sides to take part in tournaments if they wish; but the side they enter into the tournament is that side. It's either the USA, or USA A for all games. No side should be able to pick and choose what the side is designated as during the tournament and as far as I know, no side can.
The news for the ARC 2017 are that this tournament moves from February to March, and all matches will be elevated to full test status.