Speaker A thinks cardio burns thighs. Speaker B explains cardio trains the heart by briefly holding your breath during gentle activity like walking, which forces your heart to adapt and work more efficiently with less oxygen.
How is that gonna benefit my cardio? When I run, my tights burn. It's not gonna burn my tights.
Where's your heart? Here. What does cardio mean? Cardio means heart, not thighs. So when you do anaerobic exercise, i.e., holding your breath whilst doing something simple, gentle like walking, you actually exercise your heart. Because you're shutting oxygen supply. When you take away oxygen from the heart for a brief moment, makes it adapt and get stronger and more resilient to having less oxygen. It becomes efficient at burning that fuel to make energy. That's how you make a stronger, fitter heart.