This is where people like Grusch are way over their heads technically, and the wild speculation is cringe inducing when the the usual buzzwords get thrown around. "Higher Dimensions" "Alternate Realities" "Wormholes" etc do not work the way pop-sci uses them in movies and such. So many people have been immersed in science-fiction and superhero movies for so long they just hear "alternate dimensions" and imagine a plane of reality where another version of superman comes from.
People using anything from String Theory to explain how supposed ET tech works are on very shaky ground. As you touched on, it has really failed to deliver on the many promises made around it. Here is a good quick example:
Source: https://www.youtube.com/watch?v=eOvqJwgY8ow
. Weinstein is not alone, I have seen many similar sentiments from various physicists (but with a more tempered tone. Weinstein has a personal axe to grind against String Theory and he's a bit more blunt than most).
People like Grusch just throw a word salad of technical terms at people in hopes to confuses or impress the layman. By definition, some sort of advanced non-human technology that wildly defies the known laws of physics leverages an understanding of physics and engineering that is beyond current human understanding and capability. So don't conjecture on it in the first place. IMO it hurts his credibility to even speculate.
Better to just say "by unknown means" or "beyond current human capabilities" when discussing the details, lest one sounds like an ancient Roman explaining how a Harrier jet works. Not saying that guy could not never grasp the math and physics if it were explained correctly to him, but his best guesses based on his paradigm of the natural world are going to be humorous as best.
My hunch is that ALL of the guesses we make about how it works would be way off base, but obviously that's just wild speculation as well