The Modern SRD is open to anybody who wants to use it (much in the same way that the System Reference Document is).
The Open Game License allows people other than the original publisher to use content declared as "Open" (or not declared as "Product Identity"), provided that the publisher publishes the product under that license.
Contents of the System Reference Document (SRD), Modern System Reference Document (MSRD), and Pathfinder Reference Document (PRD) are available for anyone to use. Since other publishers could not refer to the D&D books or game directly by name (depending on which license was involved), the SRD provided a way for the other publishers to make a reference to the written rules.
Contents published as "Product Identity" are not "open" and cannot be re-used. For D&D 3.x, that was most of the books other than the Players' Handbook, Dungeon Master's Guide, Monster Manual, and Unearthed Arcana... nobody other than Wizards of the Coast can use the contents of MM2, MM3, MM4, MM5, etc.
Whatever modern supplements there are beyond what is in the MSRD would need to be checked for "Product Identity". I would expect that NONE of the contents of those books are "open". If that is accurate, while Paizo may do their own take on a given sub-genre of Modern, they would likely take pains to make it different from the ones already published ... both in order to avoid any legal hassles from touching content that is not "open", and from a desire to respect the generosity that made ANY content open and available.