Apple only recommends what they have exhaustively tested in Macs. They do not test in discontinued models. This card was issued for the 2010 model, and they have not tested it an anything else. They also do not recommend it for the [essentially unchanged] 2012 model.
But your friends here in the Apple discussions community can assure you it works in all models Mac Pro. If installed in Mac Pro 1,1 or 2,1 , it will run slightly slower than its published benchmarks due to the PCIe 1 slots. In the Mac Pro 3,1 2008 model, it runs at full speed.
If you look at the questions posted in the online store, you will also see many who say, "Works in my older Mac Pro no problem."
you are getting bad info. and who/where and linknif you want more feedback.
pcie 2.0 you have. apple does not do a real technical spec write up either, and misleading
there is a tech note about audiio over mdp or hdmi I have the note and url, as does Grant.