That's what the piece of paper says. Don't know why it says that, perhaps the steady level of power diminishes the life of the bulb/module faster than the declining level from the alkalines.
It's nothing new, that text has been used with their incandescent products for many years now, it's the very same text.
On the other hand, the NiMH batteries do seem to last a little longer than the alkalines in these lights.