I just did this question yesterday and thought the same thing. It's like a question I saw earlier where someone posted a practice problem with an answer of "electrically gated channels" when virtually every scientist says "voltage-gated." The endoplasmic reticulum folds and cleaves proteins, in this case insulin, and ensures proper stereoconfiguration (is that the right word?) aka quaternary structure (C-C disulfide bonds). If I should've said tertiary structure, let me know. The golgi packages the protein, with a few more modifications like you've mentioned. But cleavage primarily takes place in the ER.
Or if you're the AAMC, the majestic "endomembrane system"....
