Is this an example of a category error?

Sorry, Jim. I answered from my phone on the road and could not post detail. I also might have misread the argument in haste. So here is a more detailed answer.

I initially said equivocation with the concept of "exists," because although a rock and "truth" both exist, they "exist" in very different ways. But that really isn't the argument here. I think we would need a materialist to argue this. The argument as it is laid out seems reasonable. I would guess the materialist would argue with premise #1 in that "truth" and other concepts are products of a human mind, which is ultimately the result of material interactions. Again, I am spitballing here. I am not sure how a materialist would respond.

“To say of what is that it is not, or of what is not that it is, is false, while to say of what is that it is, and of what is not that it is not, is true”.

Aristotle

By the above definition of metaphysical truth, the OP's proposed argument against materialism would have to be false.

false premise, equivocation, affirming the disjunct, etc.

Read the conclusion by substituting material for false, i.e. the conclusion might read "Therefore, materialism is material".    So this would conclude a belief in material has the same properties as material.   So does a thought have physical properties in the same way material (like a car) does?   

But there's another aspect of this.....   again, based on truth being the opposite of false.   Perhaps there are two possible false equivocation fallacies.   Keeping the definitions of immaterial in mind, is we accept Merriam-Webster as truth, and the definitions are:
1  : of no substantial consequence : unimportant 
2  : not consisting of matter : incorporeal 

If we assume the premise 'Truth is immaterial' refers to definition 1, 'unimportant', then if we accept false as the opposite of true, then false becomes 'important', so the conclusion becomes 'Therefore materialism is important'.     So by this, fine, I accept that materialism is important, as a value system (materialism) can be important or unimportant.

Now if we assume 'immaterial' in the premise refers to definition 2, 'incorporeal', a similar flipping of the conclusion happens.   Namely materialism is corporeal.   By this definition, it circles back to the first sentences above equating a value system to something that exists in 'bodily form', physical.