Should it be called a prequel? Maybe. Is it possible to call it a prequel in the context of the Star Wars franchise? No.
Also, I think spin-off is a much better descriptor for it than stand-alone. Has either of these terms been used officially?
Spinoff or Anthology seem reasonable enough to me. But I do remember it being referred to as a stand-alone film quite a bit, and when I took my gf who had never seen anything Star Wars, she had no clue what was going on.