Did you just equate space to vacuum? Did you just imply transparent media are outside space?
BTW, spacetime can be bent, compressed, stretched, it can be squeezed into a singularity. Is there any solid rule that states it can't, given the right conditions, have discontinuities?
MM were looking for THE medium through which EM waves propagate. They made some completely wrong assumptions as to the expected nature of that medium. Yes, what they assumed it to be doesn't exist. Which doesn't mean the medium doesn't exist, merely that it's quite different than their expectations.
It may be boiling down to a semantic argument - which doesn't mean the argument is invalid or wrong. Meanwhile refusing to accept an argument may be correct merely on basis that it may be semantic is outright wrong.