These alone are sufficient to know why the goblins cannot be ambushed. The adventure basically starts with the ambush scenario. It only gives players a single turn to notice the goblins before they attack. Even if they notice the goblins, they won't get another turn to hide from them, because at that point, the goblins are already aware of a threat and attack.
If I'm being honest, this isn't helping me to "not overthink it."
The Goblin Ambush does not start with "roll initiative." Why are we talking in "turns" before the encounter actually begins? Are there "turns" outside of combat?
The book suggests the characters are free to explore the area, and the book also suggests, to me, that only when they "inspect the scene closer," the encounter is triggered and everyone rolls initiative.
The way it was written, implied to me that they've been traveling for a day and a half or so, and the Goblin Ambush section should be imagined as its own "encounter map" as it were. Once there, the goblins have already seen you. Therefore, they can't be surprised.
So yeah it will also get confusing the moment you try to understand the stealth rules "RAW". So it's better not to try to figure out how they are actually meant and rather try to figure what the most fun for your group is. And that's best done by talking with them rather than reading the rules thoroughly.
And as for this, the problem was that it was a group I didn't know well. Part of the house rules, that they agreed to, were that if there was a disagreement about a rule, the DM would make a final ruling and the game would move on, and we would discuss it out of session (which is why I'm here now, because this will inevitably come up again). Although this was agreed to, another player was essentially controlling the other player's halfling and telling them to "stealth, roll for stealth, ok now hide!" sort of thing. The not-halfling-rogue player, would not continue playing "unless 'they' got to stealth and a bonus hide action" because "he couldn't have fun if we weren't playing by the rules." (He, too, had never played, and his understanding of these things was coming solely from watching Roll20 DND sessions--he had never read the PHB at all). So as a new DM it put me on the spot very suddenly and in a really awkward position, where everyone would have been happy to move on--except one player. So I am trying to learn this rule properly/in a way that I can apply it unambiguously to the players. "The characters cannot surprise the goblins BECAUSE...." would have been super helpful from the book.