Got a response from AMD about PDEP and SAM:

'Smart Access Memory does not depend on the performance of the PDEP instruction.'

The speculation about accelerated PDEP on Zen3 being a factor seems to not be the case.

— π·π‘Ÿ. πΌπ‘Žπ‘› πΆπ‘’π‘‘π‘Ÿπ‘’π‘ π‘  (@IanCutress) December 11, 2020