All of the breaker manufactures have published statements like that. Yes, they may be able to void the warranty, but that does not make the use of "classified" breakers a code violation. The "warranty" issue is a legal one, but I have been told that it can be voided if a contractor installs the classified breaker, but not if the end user of the panel installs the "classified" breaker. (is our resident attorney still around?) This is a perfect example of a "manufacturer's instruction" that 110.3(B) does not require you to follow.