ISO directives part 2, 8th edition, subclause 15.5.2 require ("shall be introduced by the following wording") this intro text:

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

So, we can't change the text. (Plus, "content" is singular here, so it seems the grammar is actually correct; "their" just means the (single) content has multiple owners.)

The grammar is correct. The referenced documents' content is relevant to C++, and their content constitutes requirements of the C++ standard. (It would be incorrect to say their content are relevant, and also wrong to say that content constitute something).