I think there are two issues here. First, when I say "kind of thing", I mean this in an ordinary sense, as if "things" include pennies, tables, chairs, bricks, and so on. But I think you're interpreting this as if I'm talking about some form of ontological primitive.
Second, you are trying to apply this to materialism per se, but I'm talking about the term "energy" as it applies to science.
Let's go back to an older example, and put ontology to the side, and materialism as well. Pretend that regardless of what the "ultimate nature" of the universe is, I get to call something a thing if I can touch it.
Now then. Energy is not a type of thing.
A concrete block is a thing, though. I can touch it. A concrete block is also energy; Einstein shocked us with that one. So this thing is energy.
But if I put the block on a shelf, there is energy stored by the fact that the block is on the shelf. That energy is not something I can touch; it is not a thing. It is, instead, a combination of mass and height in a gravitational well.
There is no microscope I can use to see the gravitational potential energy of that concrete block on that shelf. I cannot touch that energy. But that energy is still necessarily energy, because I can use the fact that the block is on that shelf to perform work.
All sorts of abstract quantities that are not "things" I can touch or see in microscopes are "energy"; basically, any capability to do physical work, be it a concrete block or the height of an object on a shelf, is energy, and that includes a lot of "non-things".
When I say energy is not a kind of thing, this is all I mean. Energy is an abstraction.