I have come to the conclusion that the difference between science fiction and fantasy is that in science fiction matters can be understood, while in fantasy matters can not be understood.
Now the understanding need not be perfect, in fact any understanding is most likely to be imperfect, but at the least one can come rather close. But when understanding is not possible you’re pretty much stuck.
This of course has consequences. Such as when a story has science fictional trappings, but the reader has no real way to understand how things work. When such is the case the story is fantasy.
By the same token, when a story has fantasy trappings, but the reader can to some extent understand what is going on and how things work on a scientific basis, the tale is science fiction.
I mention this because after some reading and thinking I have come to the conclusion that the Ærth of Mythus is a science fiction setting in so far as it can be understood scientifically, even though it exhibits characteristics of fantasy.