When people refer to 'Pi 3' they mean 'Pi 3 model B' because there has only been a 'Model B' version of the Pi 3 released so far.
So when the Audio card documentations says compatible with "Pi 2,3,A+,B+" they mean Pi 3 Model B.
The 'Model B' text resulted from some engineers need to refer to a 30-year old computer, it isn't really relevant, although
there might be a 'Model A' produced in the future. In the Pi ecosystem 'Model A' has slightly less functionality (or slightly different functionality)
compared to Model B, and are unfortunately not necessarily chronological despite what users may expect.