Wait what which chap does it say that he was birthed by an orc?
In any for now this is just my conclusion from what have been show on the manga
I'm so curious to see the thought process behind that theory, not saying as an insult, but it's a really interesting take on it
It's very easy:
1- He was abandoned in the road leading to the Ork camp
2- the magic fabric was said to exchange the content of the bellies of the two that sleep together
3- So Jonak mother was abducted when was already pregnant but the Orks didn't notice so they made the magic exchange and and Ork mother gave birth to Jonak ( so the Ork mother nurtured Jonak inside her belly and that made him stronger than most men and he only got diarrhea from eating beasts and the dragon venom doesn't work much on him) ( although the venom thing could just be because he is Again mate)
Jonak mother must have suffered a lot, she was pregnant but her baby ended in an Ork belly.
Maybe she got pregnant young to avoid being abducted but it happened to her anyway.
Maybe Jonak is stronger because he was birthed by an Ork?