Why does it not save them? Where do you get your info?

“Upon activation of the Dead Ringer, the Cloak meter will immediately drop to 50%, and the Spy will receive a 3 second speed boost. The Spy takes 75% reduced damage for the initial attack that activates the cloak and will gain damage resistance depending on how much time is left for his cloak (from a maximum of 65% to a minimum of 20% after 3 seconds).”

The Enforcer also states nowhere that it penetrates a Dead Ringer:

“The weapon's most notable perk, however, is its ability to pierce damage resistances: the Enforcer deals full damage to enemies affected by a bullet-type Vaccinator Übercharge, Heavies spun up below half health with the Natascha or Brass Beast, the Battalion's Backup rage, the Fists of Steel, Spies' invisibility passive resistance, and Sentry Guns under control of the Wrangler.”

I think you’re mixing it up with this:

“While cloaked, the Spy receives 20% less damage from all sources.”

The Enforcer ignores the cloak’s natural 20% less damage from all sources, but the Dead Ringer still has its own damage reductions.

Please do not spread misinformation.