Here’s the distinction: What follows But is the author’s main point. What follows though is a subordinate point.
(a) I would follow you anywhere in the world you’d care to go. But I don’t trust you.
(b) I would follow you anywhere in the world you’d care to go, though I don’t trust you.
Clear enough? In (a), the author won’t be following, because the distrust is too much. In (b), the author distrusts but is going to follow anyhow."Clear enough?" Absolutely not. I have no idea why Metcalf believes that in example (a), the "But" expresses sufficient distrust to negate the preceding avowal. He seems to read it as meaning "I would follow you if I trusted you," but for me, the sentiment is the same in both versions: I would follow you anywhere, but (or though) not blindly. But maybe this is one of those distinctions I didn't learn young enough; is it one most people recognize?