For foldr, I mean that specific implementation - which fully prioritizes performance and uses several levels of unwrapping, etc. - is hard to reuse for functions we wish to maintain. Using List.foldr is fine (when applicable, which is not always), but copy-pasting its implementation to adapt it to our need isn't, from a maintenance point of view.
On the topic of continuation-passing style, I don't personally believe that the solution I've highlighted uses CPS. At least from my shallow understanding, CPS uses functions, which my solution doesn't. CPS can easily emulate TRMC, but TRMC works differently (and is more performant though is applicable to less situations).
My understanding of CPS is maybe shallow, so I could be missing some understanding, but to me they're different and therefore not part of my explanation.
> Continuations provide the structure to hold your "holes" until they can be filled in
That's one mental model to view it, but there isn't actually any "real" hole to fill with CPS, whereas with TRMC there is (at least for data construction).