Seems internally inconsistent. I may or may not have read that story a long time ago, (seems like something that probably was in Analog or F&SF) but according to the description on Wikipedia, the aliens are overconfident because "they can detect no use of gravity manipulation". But if they are only using matchlocks and black powder, how would they even begin to sense anything about Earth's society remotely? Why would basic antigravity technology be enough for viable space travel any more than rockets were on Earth in the 13th century?
In general, if there was an easy way to travel between stars that we'd missed, then visitors would be everywhere, and in fact would probably have prevented humans from ever evolving undisturbed in the first place. You may say "well, what about UFOs", but as others have pointed out, society has changed recently to where billions of people have high quality cameras with them every minute of the day, and pictures of UFOs, bigfoot, etc. are still blurry and inconclusive. If they were real, we might have such pictures, but archaeologists and astronomers would have copious evidence too. It wouldn't be just hints at the fringe.
The premise is that antigravity is ridiculously obvious & simple to discover for almost any intelligent species, and we somehow missed it.