And the guy who discovered the first evidence of the Big Bang was a priest. But both facts are irrelevant. A thing is not more or less valid because of who believes it; the only thing that changes the validity of a claim is evidence about why the claim is (or isn't) true.

"Mysticism is a personal, subjective framework for insight and self-improvement that doesn't have to follow the rules of the scientific method."

Anything that makes claims about how the world works does, in fact, need to follow the scientific method. Why? Because it's a process that's literally designed to counteract the most common mistakes and biases that humans make in order to find the truth, and because so far, it's been the most accurate and effective method of predicting the future.

If you want self-improvement or mindfulness, that's fine and personal, but making claims about the nature of the universe isn't a personal opinion, it's a statement of fact about things external to yourself that is either true or false irrespective of your own experiences.