Am I right that your objection is "The articles on marxists.org are wholly unsuitable for inclusion in wikipedia under the titles they have on that site."
That's right.
I've yet to see anyone disagree with you on this.
I agree.
That might be a diplomatic way to approach them--just say that all of their stuff is welcome, if titled "Marxist views on X..." or similar, and that some information from those articles might also make its way into the "X..." articles themselves, but not most of it.