I don't know why you think that or get that info from but the S90 with the FIX90 housing is probably going to turn out to be superb for wide angle photography. FIX is supplying adapters for:
67mm threaded (Inon 100WAL and 100WAL with dome and Fisheye UWL-04-67)
52mm threaded (various including ghe Fisheye UWL-04-52
28AD bayonet (Inon 100AD-FE and same with the dome converter and the 165AD-FE)
ST bayonet (Fisheye UWL-04-ST)
Multiple wide angle and fisheye lenses have been shown to be compatible by FIX and those of us with the camera and housing are now getting the housings and adapters in our hands. Yes, this may be THE rig for wide angle in this class. No other camera on the market and housing combination allows the total control and multiple lens compatibility this one does.
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Photo showing Inon fisheye dome lens on the Canon S90 with FIX90 housing, courtesy of Backscatter Photo:
I will shoot the Inon 165AD-FE with my FIX90 and it will work and it will give the full 165 degree FOV. I will use my DIY adapter to the FIX 67mm adapter kit.
No matter what you get, good luck with it.
N, Saints Win!