A couple of things, IMO. First (not necessarily in importance) the internal buffer pad helped to reduce recoil. Second, the roller delayed blowback system which also mitigated recoil. This was a feature that H&K included in the G3 rifle and MP5 subgun which, IIRC, they "borrowed" from the MG42 machine gun. The pistol's lower bore axis might have helped as well in relation to muzzle flip.

I think my S&W 4506-1 is quite soft shooting but I attribute that to its weight.

The P9S was an all metal pistol (save for the trigger guard), so it wasn't a light weight firearm either, but not quite as heavy as your 4506.

Harvey