Isn't it clearly stated that Emilia did it unknowingly and DP's did not react because she wasn't technically hostile knowingly? Not to mention Reinhard wasn't putting any guard against her. Plus his DP's are weakened.
There are many reasons why Emilia was able to achieve that including PiS and Tappei just wanting without care.
――If this had been a betrayal from Emilia’s true heart, Reinhardt could have blocked it.
Were it an attack holding hostility and intent to kill, Reinhardt’s intuition would not be evaded. Or, in the case his Divine Protections were working properly, sure enough, Reinhardt’s defense would not have been pierced either.
But this place was Pandemonium, and Emilia herself could not make up her own mind―― That had, in Reinhardt, left a fatal gap.
Let's close this. There are a crazy amount of reasons why he couldn't block it before we even consider giving her that feat.