That's a fair point, and for writers of pseudo-medieval or other pre-industrial fantasy, the topic of quantum physics probably doesn't come up much. For those writing modern fantasy, however, it seems to come up a great deal. And there, I find it particularly egregious, as it's often used to explain or justify the existence of magic. Usually, it is in the form of a wise wizard or scholar-type character saying something along the lines of "Science has proven . . ." Such as "Science has proven that the mind shapes reality." Or "Science has proven that there are an infinite number of worlds." This bugs me. Partly because I have a problem with the authoritative phrase "Science has proven" unless it's followed by "that this theory is accurate enough for now." Science is all about testing theories, and confirming that those theories work, at least within the boundaries of the experiment. No scientific theory is absolute, and even well-founded, well-tested, effective theories are subject to revision. Newtonian mechanics is a great theory, describing everything from bridges, to cars, to the orbits of the planets. But once you start to approach the speed of light, it begins to fall apart. Einsteinian special relativity is also a great theory, but who knows where and how it might break down. It hasn't been around even as long as Newtonian mechanics had been by the time we started discovering the flaws.
But laying aside the problems with the phrase "Science has proven," when it's followed by something that's either flat-out wrong, or a fringe theory, or even a respectable, but hardly universal, interpretation, I find it incredibly jarring. I have a hard time respecting the character who says it, as he's just shown that he's either ignorant, or lying by presenting his preferred theory as fact (another of my pet peeves). Given that the author often wants us to accept this character as the voice of authority, that can make for difficult reading.
And the real question is why. Why does the author believe that magic needs a justification? Or alternate worlds, for that matter? I'd be perfectly happy to suspend my disbelief, and accept the existence of either of those. Trying to justify their existence is not just unnecessary, but can do a great deal to ruin the mystery and wonder of them in the first place (like Lucas's midichlorians did in the first episode of the Star Wars prequels--discussed in the link). Some things it's best to leave unexplained.