It's more just an issue of using the right language of "might" and "maybe".
Scientists will never make a definitive statement of fact unless there is sufficient empirical evidence to support it. In science, you need to call a fact a fact, and a hypothesis a hypothesis, and not confuse the two. Perfectly fine to come up with a theory or hypothesis, but while that theory remains unproven, it should not be talked about as if it were a fact.
In ME/CFS I don't believe anyone can make definitive factual statements along the lines of: "in your brain, you've got A which is causing B, then B interacts with C to create D, and D has a feedback loop which operates to cause E", but Joshua comes up with elaborate models of what is going on in ME/CFS just like that, and states it as if it were a proven fact, rather than a speculative theory. So it is an issue of language.
I think it's fine to try Joshua's protocols, because ME/CFS treatment protocols are a bit like lottery tickets: you have a tiny chance of winning, but no chance whatsoever if you do not buy the ticket! So I think you are always better off trying something, rather than not trying something (unless it has risks associated with it).
Regarding Dr Markov: yes, he really should be saying something like: "we have a theory that ME/CFS is caused by kidney dysbiosis, and we have considerable evidence to support that theory". Because in science, until other scientists have independently confirmed that the theory is true, it still is considered a hypothesis by the scientific community. Nobody will believe Dr Markov until other scientists have confirmed his results. And if they fail to confirm his results, then his theory falls.