Contrast that with Three Body where the main plot driver revolves around FTL communication with indestructible intelligent nano-scale machines that can be anywhere any time, then gets progressively less realistic from there. I did enjoy the thread about the human ships going extra-solar, though, which was closer to Expanse in terms of Hard SF meeting a magical element (+dimensional travel).
What really irked me about The Three Body Problem was the way it blithely introduced faster-than-light communication as a plot point -- a key plot point! -- without any hint of a nod to the fact that this is impossible as far as we currently know. In fact, quite the opposite, the speed of light is taken to be a hard limit elsewhere in the story and that plays a key role too. So which is it?
I think it can be okay to invent magical tech in hard SF, if and only if you clearly distinguish the magical bits and work through all the ramifications logically. A great example is the “bobbles” in Vernor Vinge’s The Peace War and Marooned in Realtime.
They are not nano-scale, they are planet-scale, 11-dimensional computers projected into 3-dimensional space.