That tumbling action has a realy minor impact on hardening metal.
The reason why is that the molecules and atoms deeper inside the metal stay
as they are… soft.
The only impact (as small as it is) is at the very top of the surface and
even that is of small importancy.
A blow of a hammer will penatrate the whole area changing the structure of
the metal, changing the hardness of the metal.
I agree but you forgot one.
Precipitation hardening by heat is a well knowen way to harden metal but
not pure metals because there is nothing to precipitate.