You say that, but during Dragon Soul it was commonly agreed that the Blazing Drake had a 4% drop chance on 10 man and a 10% drop chance on 25 man. Whether this is true can't be objectively proven or disproven, but take a look at the Wowhead data:
10 man:
25 man:
The sample sizes are quite high so it's reasonable to assume that the mount does in fact drop (or has dropped) more often on 25 man. To my knowledge there's never been an official statement from Blizzard stating that all mounts have the same drop chance across all modes (or any conclusive statement about mount drop rates at all), so saying something is misinformation is a
very bold claim.