People ---> Peoples ... !?!

I'm reading a very interesting book, written by an American guy. He is well educated and has some background in English grammar. So, I guess he knows (or pretends very well) what he is doing.

Consistently he is using plural forms of words which usually you can see written the same when used as singular and when used as plural; example: Food becomes foods, fish becomes fishes and people turns into peoples.

How common is it?

But this is not all. In the next paragraph I see a vague use of the grammatical number.

It would be nice to know what you think:

"Perhaps the commonest motive for genocide arises when a militarily stronger people attemp to

occupy the land of a weaker people, who resist."

If he is talking about people as a singular noun ("a stronger people") why isn't he also using attemps (i.e., "he attemps") instead of attemp (i.e., "they attemp")?

Thanks for the advice and have a nice day.

( ;