Hi
Link the the relevant page on Ikelite’s site is here.
According to Ikelite info, the on camera flash can be used in their housing and there is a diffuser that assists with using the on camera flash.
That said, from the pictures of the housing I think the port body would block the camera’s flash from hitting close subjects. Maybe the diffuser resolves this to some extent, but I can’t see how macro shots would be possible with the camera’s flash. That said, the same problem occurs at some point with Canon’s housings too - the port body blocks the on-camera flash for close macro shots.
If you think you will be able to get a strobe at some point in the future, I think there are benefits to getting the ikelite housing - it’s more robust, has a better designed (IMHO) seal than Canon housings, and the control glands are almost certainly better sealed.
My 4c worth!