I think one extremely potent fact for that case is the existence of sorcery. When there are hundreds of people running around that can fly, make themselves immune to physical harm, destroy cities, etc. etc., all by just speaking Words... Really its difficult to argue there is no 'metaphysics' in that case.
As you mentioned, some can even physically summon entities from another plane.
Yatwer makes a literal appearance to Sorweel.
The ground talks/eats around Prosperian.
There's just a whole lot in Earwa that is magical. A whole lot more than we see in modern times at the very least. And that's the point - right? Bakker has stated multiple times that he built Earwa to be the world of our ancestors. A world where the world ends at the edge of the map, where angels and demons and gods rule from on high. So, are the Earwani any more convinced of Gods and Demons than our ancestors? Heck, some of us probably have neighbors, friends, and family members today that are equally convinced.
Speaking with authority about that which cannot be seen or proven is exactly how religion always works.
So, is Earwa religion any different than modern religion?
I submit that it is not. Religious people who brake the codes of their gods/leaders truly believe themselves damned. Many religions offer avenues for people to redeem themselves from immanent damnation - from the mundane "say these words and you're free" to more extreme bodily mutilation. What makes Earwa's religion seem outside of what modern day people do, let alone people from 100, 500, 1000, 2500 years ago.