To be honest, with a few years' hindsight, I think it might have been a bit excessive. And obviously, as you say, nobody's going to show up to your door and arrest you if you do something that isn't listed there. (In fact, I've been a player in games where the DM went off-book and it went great!)
That said, the thinking is this: As a DM, you're making up a lot on the fly. And I know from experience, it's pretty easy to come up with an idea that seems good, only to realize you've backed yourself into a corner or stalled the game's momentum. It's especially true in a no-prep one-shot like Escape from Dino Island. So the goal is to give DMs really robust guard rails and encourage them to think hard before ignoring them. "How can I twist this reveal to make it 'mysterious'?" or "Wait, does this NPC I'm introducing have something useful to offer and a personal goal?" will almost always result in more compelling situations. When it doesn't, whatever, I'm not your boss! Ignore me!
Also, speaking purely from personal preference, I've always found the way Powered by the Apocalypse games structure the DM section as "rules" rather than "guidelines" (Ours isn't the first game to do this!) to be freeing in a counterintuitive way — it lets me feel like I'm playing the game too, rather than "creating a story" or whatever and it makes it easier to feel comfortable hitting the PCs hard.
If we were to make a revised or expanded edition, we'd probably rework them to be more clearly strong recommendations, mostly because I don't really think a game needs to adopt a bossy tone, even when it's understood that players have complete freedom to do as they please.
Hope this clarifies things! And thanks for playing — glad you had fun with the game!