Like I said earlier, it's kind of a semantics issue at some point.

Yes, theoretical physics is still physics insofar as it's rooted in the current physical model, and pushing the boundaries of knowledge. It's just that there's a blurry line between talking about theoretical physics, which is more properly called hypothetical physics, and experimental physics.

To the extent that a hypothesis produces a prediction that can be observed, then it is physics. To the extent that physicists work on problems which yield no measurable predictions... well... They are certainly pushing the boundaries of knowledge, and that is a part of the progress of the scientific method. However, to the extent that certain fields, like string theory, have not yielded any results despite decades of work on them, it's at least worth questioning if it should be called physics. It's all semantics, though.

To the extent that another universe can be observed by it's subtle effects on our universe, then it's another blurry line. It's all a matter of what you consider the universe. Just because physics can't deal with non-observable properties, doesn't mean those properties are not worth investigating.

The bottom line is that, no matter what you call it, if measurable predictions can be made about it, then it's physics. It's odd to suggest that we can use physics to describe another universe, but it's all language. It doesn't change the observation.