Good Morning Michael.
Your NSX already has a diversity antenna system incidentally - there is the obvious one (the mast) and another one in the rear hatch glass.
What it really means by diversity in this instance, is a system that can select the best available detectable signals and use that as the preferred input. In cars with dual antennas like the NSX, the tuner simply uses the stronger signal. With modern electronic units, these have additional features which can enhance multiple inputs on a single input & again select the most prominent.
I'm not sure this includes XM in a single antenna but I could be wrong on that front.