> ...is this because "/[2]" was thought to be impossible, or a 1.0
> compatiblity issue, but /[2] certainly exists here.

No. There is ever at most one / node in a tree. / is never an element
node, it selects document nodes (called root nodes in xpath 1) in your
example there is one / node which has three children. /*[2] would select
the second element child of / (which is the second foo element.