To be more accurate, 们 isn't a plural marker more because of the fact that it's not productive[1], rather than the fact that Chinese doesn't have declension. If 们 were able to be suffixed to any noun to make it plural, then you could consider it to be a plural marker, even though the noun isn't technically declined. That's not the case anyway though, since 们 can only be used with a closed set of pronouns or in a limited way to refer to groups represented by the noun its attached to (in this sense it's more of a metonymic[2] marker rather than a plural marker). For example, 白宮们 can be used to translate "the White House" when it refers to the President and his administration, and cannot be used to mean "white houses".
[1] https://en.wikipedia.org/wiki/Productivity_(linguistics) [2] https://en.wikipedia.org/wiki/Metonymy