The Mechanics of Whirlwind Kick's Movement Technique
Effect: You fly your speed. The first square of this movement doesn’t provoke opportunity attacks.
Powers in 4e do what they say, nothing more nothing less. Thus you are free from provoking only for the first square of movement.
How to use whirlwind kick and avoid provoking
As you say, against one enemy you can use that first square to move diagonally up and back to leave their range and then fly where you want to go from there.If you have more enemies near you such as 2 flanking, the 1 square still help to limit it to only one opp attack vs multiple. Similarly, if you have 3 enemies adjacent and they are spaced out evenly there should still be 1 square that only exposes you to one opp attack.
Fluff has no bearing on mechanics
Fluff vs. crunch. Crunch are the rules, the parts of the power card that refer to the rules and the mechanics of the game. Fluff is any part of the powercard that is not crunch: the name of the power and the description of the power.
The descriptions of powers & items in 4e are simply that; no more, no less. The descriptions/flavor text/fluff printed on power cards has zero bearing on rules. Its there to make the power sound more fun, spice up the game, and/or help you visualize what the power is doing better.