The interpretation of that finding (like any scientific one) is that there is a general trend that that sitting position is correlated with a lack of interest. It does not mean that every person who sits like that is uninterested, and every person who sits "properly" according to you IS interested. The scientific finding just means that is is more likely that a person who sits in that position is interested. And I'd wager that your T was no more interested after she adapted her sitting position.

But I think that the point I was suggesting that you consider is that when dealing with an actual person as part of a dyad, it seems like a more productive approach to actually try to understand whether said person is said thing ("uninterested" or "ignorant" or whatever) rather than assume it, even if your assumption is grounded in some factual or general understanding about what many or most people do. I guess I think it would make more sense to understand why you make that assumption and why you interpret it with this particular T in this particular moment, and that doesn't change regardless of what the science suggests is generally true. I think when you make assumptions or interpretations about what someone is thinking or feeling, you poison your relationships.

But if all you wanted to do was manipulate your T into sitting the way you prefer, then I suppose you handled it exactly right.