The code is recursive (it is defined in terms of itself). I believe that @leftroundabout's idea about the emphasis being on function is accurate. The next slide says that
do putY (y - x)
gcdST
is
Recursive data definition
* not recursive function call
* like ones = 1 : ones
I.e. there is no recursive function call because gdcST
is not a function (it takes no argument!). This is a recursive data definition.