Very dense, yes, but stuff can be very dense and have low viscosity at the same time. Lava has a viscosity similar to peanut butter is what I've heard. You can push stuff down into it, it just requires some force to prevent the stuff from floating back to the top.

You could in principle walk on lava, either by moving quickly enough that you stay on top, or by protecting your legs enough that you could sink in maybe around knee deep where you would float.