I was reading this article and wondered about it as I’ve never heard of a female domestic mother dog doing this.
Not normally. In most modern breeding situations, the human caregivers prepare mush for the pups mixed with formula and they start eating on their own without the bitch needing to go to such lengths. In feral dogs it has been observed, as well as with dogs that are utilized in regions where dog food is not the primary food source.
The pups do indeed lick the muzzle as a stimulus to nurse, and it’s this juvenile behaviour that is retained as “kisses” later on. Basically your dog is saying: “I’m so little and cute feed me! and give me attention!”
That makes so much sense. My dog is always licking my face and lips, and nuzzling my nose. Sometimes gently biting my nose.
It sounds so gross, but when you think about it, what wolves do makes perfect baby food: warm, mushy, nutritious…
And portable. Wolves have no other good way to carry enough food back to the pups.
When your dog does that, just remember it is really trying to get you to regurgitate on it. 
Speak for yourself. 
So sez spartydog.
Poor women do the same thing - when my dad was a child growing up in impoverished nowhere Georgia, my grandmother would chew food for the little ones. (Well, not throw it up. But chew it.)
Hee! Now I think of it, I’ve taken food out of my mouth and given it to her. Pre-chewed treat. Oh, I’m gross.