I would wager that this is most likely a forgetcon, especially given a very similar answer. As you said, it goes against how the Mask of Light and Shadow operated, but also the Kaita masks.
This is a very great question, but in all honesty, Greg probably can’t or won’t answer this since that mask power was a fan creation. However, I think it still can be answered. It depends on whether a Kaita can use more than one mask power at a time. It has access to multiple mask powers, but I’m having difficulty finding confirmation that it can use all of them simultaneously. In fact, I’ve found quotes that suggest the opposite:
If the fusion is forced or unnatural, the Mask of Fusion must be activated at all times in order to remain fused. If Kaitas (and presumably other fusions) cannot access more than one mask power at a given time, then it cannot use any other power while using the Mask of Fusion.