I agree. And I think the root of the problem is that we try to understand not-self. But the actual problem is to understand the self. Once we know what the Buddha was talking about when he talked about the self, it is much easier to get a start on what it might mean for that to not be there.
I was recently invited to teach a retreat on the subject of not-self. Normally I steer away from the topic to avoid exactly these issues. So it was an interesting challenge for me to see how to frame this topic in a way that would be interesting and meaningful to a varied group of people. The talks were through theravada.pl, I’ll see if there are copies online.
This particular question was, in my view, raised and satisfactorily answered by Ven Kheminda many years ago. In brief, the phrase sabbe dhammā annattā is instruction for vipassana meditation, and in that context, seeing this is said to lead to revulsion. Nibbana can hardly be said to induce revulsion, so it cannot be included.
There’s no particularly important reason why Nibbana should not be described as a dhamma, but it seems the EBTs usually avoid doing so, and the phrase sabbe dhammā anattā is talking about something else.
