Salut VitN !
Part of the reason why this phrase may have given you so much trouble is that the English translation provided doesn't really match the meaning: Tu crois pas si bien dire would in fact be more like "You have no idea how right you are" in English. Our apologies for causing confusion - I've made sure that the lesson will be updated with the accurate translation.
With that in mind, your more literal translation is a good way of thinking of it, and it works for the intended meaning - essentially, the phrase means that without realizing it, someone has said something that was exactly right.
I hope that this is helpful, and our apologies again for the error! Do let me know if you have any more questions.
À la prochaine,