I don't think Xenonite is made of Xenon exclusively, but it is strange enough that handheld spectrometers can't deal with it. Maybe it offgasses xenon when bombarded by charged particles?
One thing the movie glossed over is how Rocky's species is in many ways much less technologically advanced than Humans. Their materials science is outstanding due to the hellish nature of their home world, but they don't have electronics. Their math and science are back in the early 20th century. They went interstellar before discovering relativity. Mostly due to the fact that the astrophage is basically magic. In the book one background character mentions offhand that the astrophage is a miracle that will solve countless problems and everybody just glares at him angrily because even though he is right it's also killing them.
On the other hand, while reading the book I had thoughts of an interstellar ferry service that collects astrophage and brings it back to Earth where it is tricked into releasing its energy into the atmosphere to warm the planet and light up cropland. Spin drives open up the entire solar system to exploitation and the astrophage is the perfect energy storage medium.