I don't see how a topological sort helps you determine where to place the computation. If an expression A is topologically before an expression B, that does not mean that A always is computed on the path to B.

For an example, consider a three-node binary tree where R is the root, A is the left child, and B is the right child. A valid topological sort is R A B, but it is not the case that whenever B is computed, A has already been computed.

I mean, it does, but that’s not what TFA’s problem statement is, and I can imagine why one may not want to immediately take apart the expression into assembly-like SSA

  let $0 = f a b in
  let $1 = g $0 c in
  ...

and instead leave some original structure in place and some tree-level simplifications available.