Theoretically true, my ass. This type of stores interoperability exists in the here and now. Has been for a long time.
An Eurofighter Typhoon can deploy guided air/air and air/dirt munitions manufactured by Lockheed Martin (Paveway), Raytheon (Paveway, AIM-120, AIM-9, AGM-65, AGM-88), postmerger-MBDA (Taurus, Meteor), ex-BAe-now-also-MBDA (Storm Shadow/SCALP, Brimstone, ASRAAM), Boeing (JDAM-equipped bombs), Saab/Bofors (RBS15), Diehl&"Friends" (IRIS-T), and this is only the stores capability that is currently operationally used. There's also a number of other stores compability that wasn't brought to ready state.
Both the Typhoon (and the F-35, and the F-16, and the F-15, and borderline-obsolescent platforms like Tornado) and the munitions it can deploy are many times more complex and capable than HIMARS or M31 missiles. And the potential consequences of a Taurus or Storm Shadow being loaded with the wrong coordinates due to a software integration error are quite in excess of that happening to a M31 pod.
If you insist on a dirtbound example, almost all current 155mm NATO platforms can fire all relevant GPS-targeted 155mm smart shells (M892, M1156, ...), no matter if the gun system is from Rheinmetal or Nexter and the shell guidance kit is from Alliant or ex-BAe.
Heck, even for unguided shells the type of accuracy a modern 155mm platform can exhibit with unguided shells -- which is considerably in excess of what ex-Soviet/Russian tube arty can do -- requires systems that are standardized across manufacturers in more than two dozen (!!) STANAG agreements, from how your meteorological sensor suite talks to your ballistics kernel, to in which format the aerodynamic properties of your shell are loaded into it.
As a matter of fact the system complexity and standardization work required to achieve this level of accuracy-with-interoperability in a pure ballistic system is considerably in excess of what you need for GPS-guided munitions.