Answer

That is a subtle question. In principle, this is true of the Written Torah as well. For example, many have written that there is no obligation to restore slavery. What the Torah says is that if slavery exists, it should be conducted in such-and-such ways. As for expounding the reason for the verse, that is a complicated topic, but in practice people do appeal to reasons all the time. I discussed this in the third book of my trilogy. As for your question, I would just say that even if we do not expound the reason for the verse, the question still remains: what does the verse state — the conclusion, or the bridge principle? If I use the example I gave in the lecture (“there is a presumption that a person does not pay before the due date”), suppose the Torah had written that one who claims to have paid before the due date is not believed. I would argue that the Torah’s point here is not that he is not believed, but that when there is a presumption, it can shift the burden of proof from the plaintiff to the defendant. One can now say that “we do not expound the reason for the verse” means that we do not look for a reason why a presumption shifts the burden of proof. That is a scriptural decree. But the application of not expounding the reason applies to the bridge principle, not to the conclusion.
You are assuming that identifying the bridge principle is itself expounding a reason, but that is not so (or at least not necessarily).