https://community.bistudio.com/wiki/SQF_syntax#Rules_of_Precedence
It should have the second highest priority. It just makes more sense that way.
I just compared it against all elements in the 10th place and they should all have lower priority than #
If you think about it, you'll come to understand my point.
Some examples:
_a = [[..................], [...................], ..............]; //100000 elements +_a#0 //it should only copy the first element
_a = [[1,1,1]]; vectorNormalized _a#0 //should work like this
_a=[true,1,2,3]; !_a#0