Train harder. The reason most people have no cardio is because they don't train hard. If you feel attacked by this, you are exposing yourself. If you do not, let's move on.

Improve your technique. Sometimes, you're not out of shape, you're just bad at Jiu-Jitsu. As you develop your efficiency, you will create the illusion that you have good cardio.

Do live wrestling rounds. I think wrestling is objectively harder to do for longer than Jiu-Jitsu. There's a reason why wrestlers come into BJJ and have crazy gas tanks and super high pace.

Do cardio work. The best cardio is being bad at Jiu-Jitsu, meaning the better you get at BJJ, the less cardio you are probably doing. Especially if you're a higher-level practitioner, cardio training is essential for taking your game to the next level.

Improve your diet/hydration. In my experience, when doing long rounds or super hard training sessions, under-eating (or not eating) beforehand can impact your ability to perform at a higher level. The same goes for hydration - make sure you're consuming electrolytes and carbs before, during, and after training.

Understand "pacing". The best grapplers in the world are world-class pacers. They can control the pace of a match/round and wear their opponent down using specific techniques. If you get someone tired enough, you can decrease their skill by a belt level or two.