> especially nuclear-armed ones.

I took "nuclear sub" to mean "nuclear powered" (as opposed to e.g. diesel-electric, which are still fairly common), which I think represents a somewhat larger set of submarines, outside the US, than just the set of all non-US "boomer" (nuclear-armed) subs. That is, attack subs without nuke-tipped missiles can still be nuclear subs because they have a nuke plant onboard for power.