It's more likely to be due to the dimmer being an older model designed to work with incandescent lamps (note the 210 W rating). As LED lamps will produce light at much lower power levels than incandescent lamps older dimmers may not be able to produce sufficiently low power levels to properly dim LED lights.
You also need LED lamps that are dimmable. (Not all of them are.) I have some but they won't dim smoothly all the way to black with a dimmer rated for
600 W of incandescent lighting. (Fitted as a replacement just before dimmable CFLs became affordable. ;)