The Chineses are a hard working people.
Is it correct to use "a" here?
Yes, use the "a."
Is no "a" incorrect then, Dave?
It's not incorrect. It's common to refer to "a people" when discussing traits of a nationality/ethnicity.
The Chinese are interesting people = Chinese people are interesting.
The Chinese are an interesting people = The Chinese (people) are an interesting set of people = The Chinese are an interesting race.