[FastFlygon]

To add to what Shining Kestrel said, Night Slash is technically superior but since Zangoose can hit Normal-typed Pokemon with Close Combat, Shadow Claw is a perfectly fine alternative. The reason it isn't mentioned on the analysis is the fact that the Smogon analyses are primarily geared towards simulator play, where practicality isn't a consideration. To slash in every not-quite-as-good option would unnecessarily clutter the analyses, which is the opposite if the professional look smogon strives for.

This isn't specifically related to your question, but if you feel like a SQSA thread would be a good addition to the NU subforum, feel free to suggest it to the NU staff on IRC. I'm aware the UU thread was spawned un part due to my own suggestion, so there's a decent chance they'll take your suggestion into account provided it's reasonsble and you put it forward in a sensible fashion.