A baby should not choke on their spit up whether on their back or when in any position.
Babies have ways to prevent choking on the spit up should it happen and have a natural gag Reflex so they don't choke on their spit up or vomit when sleeping.
Although anything can happen a baby choking on vomit or spit up is pretty unlikely because babies naturally cough up or swallow any spit up that they may have.
The reflex is a natural reflex that keeps the babies airways clear to prevent choking.