My guess would be that their warning was meant very literally. Their study only applies to filters which use ultraviolet light (many of which are probably gimmicky and do not expose the water to ultraviolet light long enough to kill cryptosporidium oocysts). Since the SteriPEN is not technically a filter, this particular CDC study does not apply to it. Using the SteriPEN according to its own instructions should yield similar results as the studies done specifically on the SteriPEN.