The NX cassette does fit the old Shimano freehub, yep but it might be missing the point a little depending on viewpoint.
It is an 11spd cassette but is stuck with the 11T top that comes with the Shimano way and all the stuff that might or might not be a downside again depending on view.
It’s not really a cheap way into the SRAM system as there’s nothing about it that makes SRAM shifting work any better than a Shimano or Suntour cassette. It’s for the OE market really.
At this point in 1×11 it’s really best to think of the shifter and RD and cassette as two separate but compatible items with different ups and downs. Shifting is largely an ergonomic decision with RD driven by shifter choice, but cassette choice is crucially a deciding factor for your drivetrain – 1×11 is quite tweaky in terms of matching ratios to riding style and that 10T top does make quite a difference to flat speeds if they’re important. No real reason not to mix shifter and cassette manufacturers to achieve an aim.