pzstd does support -d (decompression) option with a default of 4 processes, which can be set with -p. It's part of the zstd package, but I guess it's separate because it's experimental? Not sure what the difference is between zstd -T4 for compressing, and pzstd -p4 for compressing.

Anyway, for a test file I get ~125% CPU with pzstd -d, so it is able to do more work, and slightly decreases time.

Decompress zstd -d

real 0m19.125s user 0m10.888s sys 0m2.575s

Decompress pzstd -d

real 0m14.819s user 0m14.017s sys 0m4.163s