do you have some technical and historical basis to consider tango, "vals", and milonga as different dances and not just different styles or different feels?
Yes.
Without spending a bunch of time to look up a bunch of references, tango, "vals", and milonga are recoginized as different types of music. Same instruments, same musicians, same notes, etc. And yet, (again) they are recognized as different types of music.
You can easily verify that by looking at the scores on TodoTango.
Using the same logic, you can have the same dancers, the same steps, etc, and they are recognized as different types of dance.
I can't put my hands on it right now, but when the Castles invented the "Castle Walk", Irene wrote that they simply rose up rather than down at one point, while doing a dance that already existed.
Simply marketing?
"Many dances have similar STEP PATTERNS, but it is the difference, not the similarity, that identifies the "Essence" of a dance."
"Teaching 3) Knowing the essence of the dance, and being able to relate that information by demonstration, verbal communication, and "hands on" physical practice."
"You may have demonstrated, explained and written down the pattern, but you have not TAUGHT until the student LEARNS."
Skippy Blair 1994