On a fork the top cap is only for preloading the headset (and later act as a raincover...) and after you tighten the stem it is only the stem that holds the fork on.
Sorry I appreciate the purpose of a normal top cap. However yours is serving two purposes..... and I have just realised that a hydraulic brake system doesn't experience the same forces in the line as a cable system. Sorry I was being thick and have never used hydraulics.
Fork has a carbon steerer, but...
On a fork the top cap is only for preloading the headset (and later act as a raincover...) and after you tighten the stem it is only the stem that holds the fork on.