Full active stylus functionality (i.e., pressure sensitivity and precision) requires the tablet screen with the digitizer -- the XPS 9365 model.

The standard model will work with a capacitive stylus - but it's a touchscreen, not a digitizing screen.

Not sure what that means but can you answer more plainly? I have the XPS 13 that isn't the 2-1 (doesn't fold back on itself)  - if I by an active pen will it work?

Information such as this is very hard to come by on the site.  When I got my 9365 I checked what accessories were available for it which turned out to be the best way.  I even asked a online rep. and he was wrong initially.

I see nothing which seems to even hint the active pen would work with a basic XPS 13 9360, for instance and Dell does no indicate whether a display is digitized or not.

No.  The 9360 has a touchscreen -- not a pressure-sensitive digitizer.  You can use a rubber-tipped stylus as if it were your finger - but if you're looking for active stylus features, you need the XPS 9365 model.