in this answer it is stated that
For each such choice, the two balls that go into the lucky cell can be chosen in $\binom{n}{2}$ ways
(i.e. the number of ways to choose two balls from $n$ balls to go in one cell is $\binom{n}{2}$)
Then, in the comments it is stated
"Under the most reasonable model, which is that the balls were thrown one at a time towards the cells, independently, with all cells equally likely, it makes no difference whether or not balls and/or cells are distinguishable"
Why under such a model does not not matter whether the balls are indistinguishable or not?
(specifically, I thought that the $\binom{n}{2}$ would be multiplied by $2$ factorial, since for any chosen pair we could reverse the balls and have another choice.)