I am researching various models of Neutral Naturalness which involve the addition of an additional gauge group whose matter content is uncharged under SM color. Many of these theories state that their low energy dynamics are controlled by a "pure" Yang-Mills theory and then go on to say that this means there can be no light matter (and thus the lightest states are glueballs). I have yet to find an explanation of why this is the case. I am further confused because these theories all have a top partner to cancel the quadratic divergences in the Higgs mass, so they do in fact have matter content. My question is two fold:
1) What exactly is a "pure" Yang-Mills theory, and
2) why does it forbid the existence of light matter, despite the existence of a top partner?