I do believe Cassiterides is confusing a VACUUM with a lack of a MEDIUM.
At the opening of the 20th century, it was believed that no waves -- including light waves -- could propagate unless there was some medium present that could "do the waving." The medium that sound travels through is air (or water, or solids). The medium that ocean waves travel through is the surface of the ocean. Therefore, there "had" to be some medium that carried light. They called this hypothetical medium "the luminiferous aether."
No one, however, thought that the aether was a meterial medium like air. A perfect vacuum could still (according to this hypothesis) be permeated by aether, whatever it was that aether actually "was."
The Michaelson-Morley experiment was the first of many death blows to the aether hypothesis. Incidentally, according to my high school physics teacher, a Japanese theoretical physicist in the early 20th century proved (mathematically, probably using special relativity) that you don't need an aether for light to be able to propagate.