Eh. He worked with frogdesign, Paul Rand, Avie T., and a host of software people with strong opinions at NeXT. He wasn’t incapable of working with other people.
For the specific request here (Classic II and current MacBook), you might be able to build an executable with Mach-O data fork and the 68k code in the resource fork. I’m not sure offhand how to make the bundle format work for current macOS, though.
But if you can make that work, you can also add classic PowerPC support, by jumping to it via a 68K CODE stub and perhaps from a separate PEF fragment stored in a section in the Mach-O that would be ignored on current macOS.
If so, that’s a real shame. Foundation may be the single best-designed standard library for a (non-functional) language ever.