Uh, you know in some cases the thing about LED lights and dimmers is true, it depends on the driver circuitry inside. For non dimmer compatible it likely won't dim at all, or not in the same way as a incandescent would (not as smooth/in steps, dim much more rapidly, etc). Leds are quite a bit more sensitive to voltage/current then incandescent are, and the driver circuitry has to convert high voltage AC to a controlled DC level. You don't have to buy a dimmer just for LED lights, but they do make some that work better. II have quite a number of LED lights from various manufactures, not all of them work on all existing dimmers (not all "old school" dimmers are created equal either).

The sales person is likely trained to say that about the bulbs so that they don't get people coming back pissed when things don't work. They likely do not know how it all works, just what they have been trained to say. Please pick something more worthy of rage, this makes you look like a buffoon.