The bonuses don't stack
The 1st-level Desert Wind boost wind stride (Tome of Battle 56) grants a bonus only to the creature's land speed, while the 3rd-level Iron Heart stance absolute steel (Tome of Battle 66) grants the martial adept a +10 ft. enhancement bonus to the adept's speed generally.
As both are—eventually—identically named bonuses to the martial adept's land speed, the bonuses don't stack but overlap, so only one of the bonuses is realized (although were one of the bonuses to disappear, the other would remain).
That means, when the martial adept assumes the stance, the martial adept really does benefit from a +10 ft. enhancement bonus to his land speed, even though that 10 ft. bonus is applied individually to all of the martial adept's speeds that he possesses. (For example, the martial adept would gain a bonus to his swim speed if he had one, but would not suddenly and inexplicably gain, like, a burrow speed if he didn't already have one).
The stance's enhancement bonus to speed broadly prevents the boost from adding another +10 ft. enhancement bonus to the martial adept's land speed because the martial adept already has a +10 ft. enhancement bonus to land speed. (If it makes you feel any better—and it won't—because specific trumps general, the boost's specific enhancement bonus to land speed technically trumps the stance's general bonus to all speeds… not that that makes any real difference.)
To put this in a different context, for example, if an effect said that it granted a creature a +2 morale bonus on all the creature's ability scores, the creature couldn't add to that—or stack—an effect that granted the creature only a +2 morale bonus to its Strength score.