I'm quite surprised Duolingo rejected my sentence simply because I wrote "Lets" and not "Let's". In most courses forgetting the apostrophe usually gets you the word marked red, but doesn't mark the complete sentence as wrong.
We don't get to decide what gets treated as a typo (and accepted with a simple warning) and what gets rejected. It's an algorithm that determines it and we can't change how it works. If you got your answer rejected because of a typo, take note of it and write it correctly the next time.
Why not "let's help these people out"? Is there another specific equivalent to "help out" vs "help"? Because in English, the two phrases are utterly identical and interchangeable...
It's not slang - it's used interchangeably with "help", both in speech and in writing, in American English and has been for generations. It may have started as a colloquialism in England in the 1600s, where it first shows up, but is pretty firmly part of everyday speech.
Maybe you meant to say it's an idiom?
Yes, you're right, not slang but a colloquialism. There's no grammatical need for the preposition, unless you're helping your grandmother "out" of the car.