The walls and PDs are, as far as I can tell, equal now. The weird findings from the duelist are the result of him testing the walls on completely flat ground.
He mentions that walls do nothing to protect t1 pds (on completely flat ground):
https://www.youtube.com/watch?v=AIe_160lwVY
I can reproduce his findings quite easily too:
https://youtu.be/y60-_oqdyFk
Interestingly though, on even slightly bumpy ground, walls are quite effective at protecting the pd:
https://youtu.be/fsAQEd7KQz4
So for all normal maps, walls and pds interact basically as you'd expect while on crazyrush etc. walls are completely useless at protecting a pd.
I'm assuming that was a weird accident, and not something done intentionally, @Tagada ?