I don't really think of thin value betting and semi-bluffing as the same concept on different betting rounds. Thin value betting gets its positive expectation (versus checking) entirely from getting called with a worse hand. It doesn't really have positive expectation from inducing a fold the way that semi-bluffing does.
At least, that holds if you assume the opponent will always call with hands above a threshold and fold with hands below it. Which is correct play (though kindly ignore raises, for simplicity - but they don't blow up the argument). Only if they behave very strangely, e.g. folding with the very best hands and worst hands but calling with the decent ones, can I see a better connection with semi-bluffing on early streets.
I should also say my explanation assumes we're in last betting position here. If we're first to act, the logic is somewhat more complicated than I can successfully conjure up here without going back into the books - it's been many years since I played seriously.
At least, that holds if you assume the opponent will always call with hands above a threshold and fold with hands below it. Which is correct play (though kindly ignore raises, for simplicity - but they don't blow up the argument). Only if they behave very strangely, e.g. folding with the very best hands and worst hands but calling with the decent ones, can I see a better connection with semi-bluffing on early streets.