I'm going to vehemently disagree.
Partially combining the 2nd guy and the guy with the turbin's statement with my own experience. A lot of people made a lot of good points here... enough to make your point mostly moot.
You're right, but not right enough.
"Cardio" makes the breaks between the rounds seem longer to me, I know for a fact my heart rate and rate of breathing slows more in my minute off when I do supplementary conditioning than when I don't.
Your workload in a round is a variable at this point. You could be a slow, lazy, low-output guy who shells up and eats punches for three minutes, returning only one punch at a time. So we don't have enough information to validate your statement, let alone speculate on it.
The part in which you are correct is that: being pushed by a partner to produce a huge workload in the activity you are seeking to improve in is, in fact, the best form of sport specific conditioning. However, like everyone says, it can't be 100% of your workload. You skip rope, you punch the bag, you shadowbox, you hit pads, you probably do pushups and situps..... all of these things are forms of "conditioning."