This is incorrect, floatyness does not lessen horizontal blows; falling speed does not have any effect on horizontal blows whatsoever (other then slightly influencing the exact trajectory). The reason why some fast fallers like Fox die earlier horizontally than lighter floatier characters is because they have a high gravity value, which then causes them to actually sustain more knockback than some lighter characters (more gravity = more knockback). This gravity also causes them to die vertically sooner than they should with their weight and falling speed since they're sustaining more actual knockback, which is why Fox's vertical endurance is on the low end in Brawl despite still having his insane falling speed, unlike in Melee where it was among the high end (as gravity didn't effect knockback in Melee).
For gravity, it correlates with falling speed (all the space animals have high gravity values, and all the super floaties have low gravity values), though they're not completely dependent on each other, as you have the likes of Squirtle and Bowser with among the highest gravity values despite their more floaty falling speeds, while Dedede and Snake have middling gravity values despite falling as fast as the space animals (which is why those two also take horizontal hits ridiculously long despite falling much faster than the other super heavies).
Now meteor smashes and other downward hitting moves will be directly worse on fast fallers, while not being as effective on floaties.
