I do not see this as goalpost shifting so much as asking a question and believing that you misunderstood how we arrived at A, B, C, then D.
It was the players choosing to do these things in that order out of more available options, just like I said you have A-G for the starting player, H-R for the next step and S-Z for the one thereafter.
If the DM is still setting "you can select from A-G until you've done all of them", that doesn't at all look like a "sandbox" to me.
It looks like a theme park. You can go to any attraction you like, but once you've had your fill of ride C, there's no reason to go back to it.
Like...that's the core issue here. "Pick any three you want from A-G, which I the DM have prepared" doesn't strike me as a sandbox. It strikes me as a DM offering choices, certainly, but not actually a sandbox where the players hold the reins. It's still a DM-prepared, DM-curated menu. They just get somewhat more freedom.
Consider the following sliding scale of rails vs freedom:
1. Dragonlance-style railroad where players play specific premade PCs acting out a specific plot
2. More wide-ranging railroad where the PCs moment-to-moment are fully under player control, but the adventure won't change
3. Secret railroad where the players
think they have choices, but every choice is bent toward the DM's pre-planned arc
4. Soft railroad, where the PCs choose the
order of the adventures, but still have to do all of them
5. Branching paths, but only in limited ways that are all DM-prepared--DM develops each a bit, and then fills in once PCs choose
6. "Deck" of adventures, where the DM has filled the deck, but the order isn't decided by anybody
7. "Menu" of adventures, where PCs select three proverbial appetizers from a list, three entrees from a list, three sides, etc., etc.
8. "Organic" adventures, where DM solicits player ideas, then develops adventures from them for players to choose between
9. Pseudo-sandbox (often mistaken for full), world is nailed down pretty hard in advance but evolves in response to PC acts
10. Full sandbox, where PCs are fully in control of what they do, where they go, and why they care
Most of these are of course my terms. A "menu" adventure is certainly more freeform than not. But "sandbox" is on the
extreme end of freeform adventure, combining characteristics of most things in the bottom three/four of the list above. Saying that a menu of adventures isn't a sandbox doesn't mean that it's therefore a rigid and inflexible railroad. It just means that it isn't
all the way to "sandbox" yet.