What's really wild about this is that it's actually a strike-slip earthquake, based on the focal mechanism (e.g.
https://earthquake.usgs.gov/earthquakes/eventpage/us2000cmy3...) Given how big it is, I'm extremely surprised that it's stike slip. Furthermore, given the area (shallow and very near a subduction zone), anyone would immediately guess that it was a megathrust earthquake, and therefore likely to produce a tsunami. Interestingly, it's not at all.
That's likely a big part of why the tsunami watch was canceled so quickly. In addition to there being no direct detection of tsunami waves, strike slip earthquakes don't produce significant vertical movement of the seafloor (barring triggering a landslide, anyway), and therefore don't typically produce tsunamis.
However, Mw 7.9 is huge for a strike-slip earthquake. They're usually not that large. Furthermore, it's out in the oceanic crust of the incoming plate, so it's not a subduction zone earthquake at all. Instead, it's related to internal deformation of the incoming plate (albeit very near the plate boundary, so it's not exactly an intraplate earthquake either).
Large strike slip earthquakes in oceanic crust aren't unheard of, but they're quite rare. This earthquake appears similar to the large strike-slip earthquakes near Sumatra in 2012 (e.g. http://www.earthobservatory.sg/news/exceptionally-large-stri...). Definitely interesting!