First of all, people who use apple cider vinegar dilute it with water or mix it with aquaphor. It kills bacteria and fungus.
The tissue inside the mouth is different from the tissue on the lips. The skin on the lips is the thinnest on the body. The mouth is genetically made differently to hold saliva, not the lips.
Personally, if I get saliva on my own lips it makes my lips chapped. Since my nasal passages are blocked, I'm a mouth breather so saliva tends to get on my lips from open mouth while asleep. I noticed if I apply a protective barrier on my lips at night, they don't get dry like they would if saliva touched them.