If clipless pedals offer a slight increase in "efficiency", ie allow you to waste less effort, then they don't really help cardiovascular fitness. You end up riding a tad faster or farther for the same work out.
If they increase your enjoyment or motivation and as a result you ride more or harder, then they have indirect fitness benefits.
Pro cyclists don't put much effort into "pulling up" (mountain bikers pull up a tiny bit more than roadies) but at least they don't press down much on the pedal either, so they must at least be pulling their leg up to some degree. So your pulling up muscles get a bit or a workout even with platform pedals, since you at least have to lift your leg.