You are Mo, a skilled tinkerer, living with her family on a pastoral archipelago
- Everyone lives alone on their own island and when you do meet them it sounds like you haven't seen them in months.

an almost forgotten threat will swallow all.
- Everyone is well aware of the spores and 3 of the 5 inhabitants just aren't that bothered by them. The spores do "swallow all". It turns out not being that big a deal.

Face the dark truth beneath the islands’ surfaces and unravel an emotional poetic story
- None of that happened. I don't know what dark truth this refers to and the story was anything but poetic.

uncover the archipelago’s secrets.
- Nothing was secret. The characters knew about the nature of the islands from the start. The only new piece of information anyone learns is when Mo realizes everyone else was planning to leave on a boat.

I feel lied to.

Yes, their relationships are complicated, but it is also mentioned that in spite of their differences, her uncle and sister stayed because they just couldn't leave "their little Mo" behind. Forgotten is perhaps a bit much, but it does feel like they just tacitly relied on Mo and her rapport with the giants to keep the spores in check. Which is the plight of every infrastructure type job, people only notice when it no longer gets done. Whether it's a big deal of not is a matter of interpretation- accepting defeat regarding your life's sole purpose can be a massive thing (see Disco Elysium, or, more trivially, A Plague Tale: Requiem), and objectively, people displaced by climate change (which no doubt informs this game to some extent) probably won't feel it's no big deal simply because they have boats. Again, it's up to interpretation, but Mo's self-doubts found in the underground rooms seem to be just that. "What remains of you when you're gone? Nothing but dust and decaying bones. How many minutes have you wasted, to be distracted by sentimentalities? You wish you had been a heroine, but all that remains is the role of the martyr." I find that sufficiently dark. (I'm retranslating from the German subs, so it's not exact.) Doesn't it depend on the perspective though? Mo is an expert on the workings of the brothers, which the remaining surface-dwellers remain blissfully ignorant of. "Mo could hear the silence below the earth crust under the island:" They probably aren't privy to the reality Mo ponders in the underground of Boan: "The brothers are very old, but they are not immortal." At the same time, her single-mindedness clouds her perception of everyone's desire to just leave. Well, the text is an advertisement designed to make the game sound interesting. That said, I personally feel it's not entirely off the mark.Your interpretation is a valid as mine though, of course.

Not to be rude, but I think you just read well more into the game's description than anyone was ever meant to.