Newt's words seem to expect a specific skill that he just doesn't have no matter how much his parents pushed. So they somehow convince Newt that his soulmate is actually Theseus' (who does have said skill) and he's got the words so he can introduce them.

Theseus kind of goes along with it as the words sound kind of threatening/dangerous and he just wants to protect him.

So when Newt meets his soulmate (which reveals it's an honest mix up/completly innocent), he tries to introduce/match him up with Theseus. Now Theseus has to explain why Newt doesn't believe his soulmate is his soulmate and convince Newt of it as well.

The skill could be duelling, fencing, Quidditch, hunting,...