Whirlwind works just fine.

You're right that it depends on a speed advantage to function.

You're wrong a bit in how you think it works.

You're definitely wrong a bit in expecting it to destroy everyone simply because your character has a moderately high dex (e.g. 23).

There's more than one way to be faster than your opponent. There's even more than one way to have a higher dex than your opponent. The more of them you can capitalize on, the more this legacy will do for you.