I find that science fiction fans are a little concerned with "proper categorization", and often discussions get into people's own personal interpretations. I don't share the OP's definition of the dilineation between scifi and scifantasy, I'd categorize Children of Time as pretty direct scifi, but I also don't care all that much whether someone else wants to give it a different label. I don't think it's incumbent upon the author to justify the plausibility of every technological feat within our current understanding, but more power to you if you do.