I mean it's not surprising that, in firefighters, pushups would be a better predictor of cardiovascular health than a VO2 max estimation. For the treadmill test, they cut participants off at 85% of their predicted max heart rate. Whereas with 40 push-ups you're pretty much guaranteed to be at your actual max heart rate after the first 10 or 15.

In other words, push-ups are probably a better measure of cardiovascular fitness than the treadmill test, but only for the tiny percentage of the population who isn't limited by strength.