This is a common mistake. If this was correct, airplanes with asymetrical section wings wouldn't be able to fly upside down - and they can. The appendage being symetrical or asymetrical is not the real issue.

What your statement neglected is the angle of attack. Given enough angle of attack, a canted symetrical daggerboard may lift a boat out of the water (although not as efficiently as an asymetrical daggerboard) - so it behaves like a hydrofoil.

Conversely, a non-canted asymetrical foil generates only lateral lift to both sides (although not as efficiently as a symetrical foil) - so it is not a hydrofoil. It does not provide vertical lift until the boat heels.

Conclusion: the rules need a better definition of "hydrofoil". But a proper definition is quite a challenge because with a bow up atitude, a "plain vanila" vertical symetric daggerboard will provide vertical lift as the boat heels. And the hulls as well, even without heeling!

How do the A class rules define "hydrofoil"?