„Laute“ (die Saiteninstrument) ist ein weibliches Nomen. Aber „Leute“ ist ein Pluraletantum, und weil es ein Pluraletantum ist, ist es weder weiblich, noch sächlich, noch männlich, denn Nomen, die nur im Plural vorkommen, können kein Genus haben, da es nicht notwendig ist.
"Laute" (the string instrument) is a feminine noun. However "Leute" is a plural-only noun, and because it's only plural, it's neither feminine nor neuter nor masculine, because nouns that only occur in the plural form cannot have gender since it's unnecessary.
(P.S. I don't know how bad I butchered my German, but I tried)
Where do they say that?
There is a countable noun "a people" (roughly meaning "a nation") and then you might say "this people is...", but that's not a very common word.
The noun "people" that is used as a plural, on the other hand, is very common. (The one that usually takes "person" as its singular: one person, two people.)