To be fair, it's usually more along the lines of 'armed in the Roman style', which would be useful in briefly conveying the manner of fighting for a Roman audience who understand instantly what kind of armament is implied; and indeed, it does refer more to equipment than organisation, which was what really distinguished the legions. Imitation legions is something more associated with later historians who somewhat mistook the meaning of the histories in a time when Rome was coming back into focus, and something applied by CA when designing units.