It's not a hardness test mark, I know what they look like, this ain't it.
Hammers are hardened throughout, or are supposed to be.
If properly heat treated they will not dimple from hitting the firing pin and also will not break. I have many firearms that have no dimples on the hammer (FAL's, AR's, Daewoo's, Garands, M14/M1A's)
Sig said it themselves that there should not be a dimple.