The most surprising thing to me is that they can parameterize a nontrivial section of the implementation space of a function, or that such a section exists that hasn't been optimized away by the compiler.
The thing is that compilers usually quickly go to SSA form and it is not the best IR to optimize loops in. Then you fight it to extract a high-level IR and this little process makes it very easy to lose high-level information. We don't do this so we begin from a friendlier starting point.