I don't know if I totally agree with that statement. One easy way to avoid impaction problems is to feed from a clean dish or bowl or from tongs. Also, substrates like indoor/outdoor carpeting are terrible at holding humidity. A soil, decomposed granite mixture, moss, leaf litter, cypress mulch, etc does a much better job. These are also much better for burrowing herps like monitors.
That being said, I have seen croc setups where they are kept on carpet-type substrate, and they appeared to be alright. Crocs will get their humidity from the water, so I wouldn't think that would be an issue here. I'd put some plants or some sort of shelter on the land that the caiman could hide underneath though.