It's not? Better tell the Navy. They say it is routine.
I suspect this may have been the first time this route was traveled by a submarine that deep.
FYI, Guam is homeport to several subs and home-away-from-home for others. Brisbane is a routine liberty port.
Nuclear Subs have only been based at Guam since 2002.
Relative to the North Atlantic, the GIUK gaps, North Pacific, the route has to be a very rarely traveled route as nuclear submarines go.
It's certainly mapped far less well.