I think it's typical of the Thorpes. I think John is so simple-minded he's just presuming or hoping she will go along with the idea of marriage.
I can see your bafflement and one wonders how he can be so thick. My theory is John is so wrapped up in himself, all he's thinking about is her dowry and his belief she is an heiress so he will be rich.

Throughout her Bath visit, they never listen to Catherine do they ? John asks if he can call and see her family, she politely says yes-well, John will be as family ! Then to discover from Isabella, John considered this as 'favourable' encouragement to his proposal.

I'm unsure if he delieberately kept his wording vague. Even stupid people can be cunning.