Cores are not exclusive to retroarch, anyone can implement a libretro frontend and use the cores. I think some early 3ds ports were based on libretro cores since it was easy to use them (they are already stripped of most platform specific code)
Yeah, the early Nestopia port by @Steveice10 used RetroArch's Nestopia core, and I managed to get Gambatte working based off that code, but I never released it. I even had SNES9x Next partially working, but I wasn't really interested in trying to fix the weird glitches it had (it wasn't from RetroArch, it was glitchy due to my port













