There are a couple of problems you could run in to using a castable product for a kiln lid instead of IFB. First, as Norm mentioned, can a large flat disc of castable handle the expansion and contraction without cracking? Even with a metal band around it, if it cracks it's done. Lids have to be structurally sound. Second, would it insulate as well as IFB? The power needed to heat up an electric kiln is calculated based on the insulation value of the IFB's. If the castable does not insulate as well, especially in an area as important as a lid, it could affect the ability of the kiln to function normally. Third, what does it weigh? If it's heavier than IFB, then you may have to re-engineer the hinge system on the kiln.
This Kast-O-Lite product is available in lower temp versions, like 2600F and 2300F. And just like IFB, the lower the max operating temp, the better the insulating value. In looking at the specs on the Kast-O-Lite vs. IFB, the castable is heavier, more dense, and more thermally conductive than the IFB. Probably not a good choice for an electric kiln lid.
Castables definitely have their place in kiln building, however they are typically used in specific areas, rather than for the whole kiln. That said, I have seen several gas kilns made totally of castable, of catenary arch design, used for salt firing. But they do tend to crack a lot since they are monolithic, and an outer layer of insulating material goes a long way to improve their efficiency. Castable also tends to be quite expensive compared to IFB. Personally, I have used castable products to make the throat arch in a wood kiln, for lining fireboxes in soda kilns, for making the key in sprung arches.