I can tell you it wouldn't likely be memory foam, as memory foam is a lot denser and probably would impede heat dissipation.
Truthfully I don't have any idea what it really is.
Here's a little speculation not necessarily based in fact...
I have a sample of memory foam I got at a home show. It is identical in feel, weight/density, and color to the foam in the HR21.
Memory foam has a very weird feel to it. The surface has a kind of oily/sticky feel to it. It is because of the similarities that I think it might be memory foam.
Heat has always been a problem with memory foam. There are some composites and re-formulations that are supposed to be better at dissipating heat. Regardless of composition, the one in the HR21 certainly denser/heavier that the standard foam pad. Not that it matters. Any foam pad will provide thermal insulation. However, the fan is right up against the drive and will easily control the temp in the drive.
It's just an incrediblely weird place for any kind of foam pad. It can only have a limited number of functions...insulation, air flow, sound, or vibration. IMHO it can only be for noise or vibration with an outside chance of air flow control.
It's a theory.