This only holds true if you first assume that 2(2+1) is somehow “joined”. Which is where the entire argument lies.

If you make that presumption any mathematical tool would result in the answer being 1. However, I and anyone else in the “9 camp” would argue that 2(2+1) is no different to 2 * (2+1), and similarly that there is no difference between any of the following:
6 divide 2(2+1)
6 divide 2 * 3
6 * 3 divide 2
(6*3)/2
6 * (1/2) * 3

All of these are equivalent from my perspective, and the basis for that equivalence comes from the interpretation of
“Number divide number(bracket)” as interchangeable to
“number divide number times (bracket)”
Whereas people who support 1 as the answer would interpret it as
“number divide (number times (bracket))” which I would argue is not equivalent