Yeah but its not just that they are good at 1v1. Skirmish players are superior mechanically in a variety of aspects and I don't think you could simply say that they are good at 1v1's by the way they are good at killing a large number of bots very quickly and not dying.
Let me tell you why you can't beat "Rambo". It's because in order to beat it you have to be on the same general mechanical skill level as the rambo'er in question. If you, for example, had a competitive level skirmish player with an archer squad, they would easily be able to absolutely ruin the day of 1-2 or perhaps even 3 "Rambo" cav by sniping them off their horses in short order. Even if the archer unit was focused for this reason, it would cause greater damage than loss to the Rambo cav in question, or force them to ignore the unit completely. In other words:
Rambo is not a "meta" nor is it an infallible strategy. It is a strategy that ONLY EXISTS DUE TO THE MECHANICAL INCOMPETENCE of Captain's mode players.
Rambo does not work as well, if at all (to the extent you could call it the meta) against two teams of roughly equal mechanical skill.