"Certain" can also mean "some": to quote the Merriam-Webster dictionary, "of a specific but unspecified character, quantity, or degree <the house has a certain charm>". That's what duolingo's pointing at, in this case the unspecified quantity; "certo" and "certain" have similar contexts.

la gente is always singular. le persone is always plural. Both mean people. Perhaps is the ambiguity in the Italian idea of "people" - does it refer one thing or many things? We British seem to have a similar ambiguity about sheep.