For the GIRM, if the options weren't hierarchical, then wouldn't the options be more randomized? In other words, the Graduale Simplex - or even the "4th option" - would be listed before the Graduale Romanum. The options seem like a logical progression to assist music directors in choosing the right music for their circumstances. For example, maybe the Graduale Romanum would not work out for a beginner-choir, so you go down to each option until you find one that works for you.

In the GIRM, the options also use numbers. If the options weren't hierarchical, I'm wondering if letters would have been used instead. Numbers give me the impression of preference.

But I don't know if there's anything official that says that these options are really hierarchical, it's an interpretation of mine and others to say that. Sometimes, it's interesting to base an interpretation on what is said - and not said - in the GIRM.

Indeed, it doesn't say anywhere that they are hierarchical, but with all these things, it's hard for me to read it any other way.
And despite all the deductive logic and reasoning that this calamitous connundrum has endured for decades, Ben, it's also not unlikely that the lack of precise language clearly articulating heirarchical preferences was intentionally suppressed (after the council, of course) and otherwise ignored. Tucker re-openeed this "wound" in his article "Some knew" over at the Cafe.
Personally, I think the horse quite a few folks with disparate agendae agreed needed to get out of the barn right away had everything to do with enculturation. YMMV.