Why would a light fixture be incompatible with dimmer switches?
The best explanation I can think of is that many compact fluorescent(CFL) and LED bulbs are not compatible with dimmers. The reason why is

non-dimmable CFLs cannot and should not ever be used with dimmer switches.

and

To put it in layman's terms, most modern dimmer switches essentially function the same way as if you were to turn a light on and off really, really fast (about 100 times per second). (snip)
basically the problem is this: CFLs have less resistance than incandescent bulbs, which means that the floodgates are opened when they are exposed to the electrical fluctuations that a dimmer sends, and they actually can consume up to 5x the current as when they're not connected to a dimmer (and that's when the dimmer is set to max). This overheats the bulb and can actually cause it to light on fire. Bad news.

The fixture has a GU24 base (non-standard) which is going to further limit your selection of bulbs (as in your local store may only carry non-dimmable bulbs). So rather than accept liability for a potential fire hazard, they're just going to tell you not to use a dimmer at all for the light.
In theory you should be able to hook a CFL/LED friendly dimmer to the light only and install a dimmable GU24 bulb, but beware that any liability for doing so lies solely with you.