The Sequoia has the next pool closure for the beginning of December so more likely than not the pool will be open when you go.
I've been in September before when they closed the pool once but this was because they reached capacity and wouldn't allow other people in.
You can never 100% guarantee anything at Disney as things can always be subject to change due to unforeseen circumstances but I'd feel sure in your situation that the pool will be open for you when you go.
