Under <R7> subsection c you are allowed to use:
Would I be correct to assume that it truly means any nut?
Specifically are rod coupling nuts legal such as these under <R7> subsection c, for the purpose of using one nut to secure two screws, assuming I find them in a legal threaded variety?
No, these would not be legal. The couplers you have linked are functionally more similar to standoffs than nuts. The intent of the rule is to allow for reasonable replacements of existing VEX nuts with similar commodity hardware. Adding the functionality of coupling two pieces together goes beyond what is expected of a standard nut.