The sophons themselves were a piece of magic science fiction. Which I think is fine because the author really doesn’t ask you to suspend your disbelief all that much throughout the books. The star plucking is another example, as far as we know you can’t use a star to do that.

But accelerating them towards earth “at the speed of light” isn’t exactly a problem. The LHC accelerates protons to about 3 m/s less than the speed of light, and as far as the plot is concerned the sophons travelling here at the speed of light, or some tiny fraction of a percent less than the speed of light doesn’t make any difference.