Sounds like Apple. Documenting features that in reality will not work, because other software completely ignores or overrules settings.
The setting is called NSSupportsAutomaticGraphicsSwitching which suggests that the system decides which GPU to use, and apparently based on the load of openMSX the system decides to use the external one. Then if you really want to overrule that decision you need to do so explicitly, for which there are APIs but SDL2 doesn’t use it.
