Insight is a contested roll to detect lies, but that's not its only function. I used it here specifically to gain insight into motivators. When you mentioned the subject families, you were watching to see if his eyes widened in fear or he jerked in surprise or basically any body language that told you you struck gold.

And if you fail the roll, your character simply didn't notice anything. There is no need to make up falsehoods and force a player to act as he believed it all in this foolish quest against meta gaming. Unless you crit fail, which to be sure is a house rule anyways.

It is literally impossible not to meta game. You character is an extension of you, and their motivations and actions and thoughts are yours. Your knowledge and experience informs your characters actions; your imagination and creativity enable your story. If you, for instance, were raised by wolves, you would not understand a lot of concepts like justice, war, barter, falsehoods, or propriety; you therefore literally could not play a character using these concepts.

The metagame foxhunt is a trap. You can't divorce player knowledge from character knowledge and there's no point in trying, as long as they a character reason to explain it that isn't pure sophistry.