I'm hoping to clean this thread up so as to not confuse any inexperienced people who might be visiting this forum.
The point I made was that just because cosmology can be theoretical, it doesn't mean theoretical physics is cosmology. Theoretical physics is not a 'field'.
Theory is not specific to any branches of physics. Theory is a particular approach you're taking to describe the physics.
Cosmology is a specific branch of physics. You can be a theoretical cosmologist, just the same as you can be a theoretical solid state physicist. Like I said, physics is theoretical or experimental. Every specific subject in physics fits into one, or both, of these classes. Think of them as umbrella terms. Within these classes are the hundreds of different subjects.
The two will never be indistinguishable - you cannot study 'theoretical physics', unless you take it to mean everything that is theoretical in physics (which would, literally, be impossible to 'study') then it isn't a subject.
You confusion may have come from the fact that describing oneself as a theoretical physicist is perhaps a bit of a misnomer, since 'theoretical physicist' misses the actual subject one is working on.