I think it means the headphones are capable of producing a frequency that high, even if there is no use for it. The same reason why a minivan has a top speed of 120mph or 200kph, there is no reason for a minivan to ever go that fast and I think only a handful of them ever have, but they are capable of doing that.

If they purposely made them max out at 17kHz, then there could be some sounds that the headphone isn't producing that are supposed to be in the mix.