Completely forget about MkLinux. The timing is fascinating.
MkLinux was released in February 1996 whilst Copland got officially cancelled in August 1996.
So it's definitely conceivable that internally they were considering to just give up on the Copland microkernel and run it all on Linux. And maybe this was a legitimate third option to BeOS and NeXT that was never made public.
What's crazy is that MkLinux was actually Linux-on-Mach, not just a baremetal PowerPC Linux. The work they did to port Mach to PowerPC for MkLinux was then reused in the port of NeXTSTEP Mach to PowerPC. Everything was very intertwined.
Also, MkLinux wasn't that stable. I experimented a bit with it at the time and it wasn't really ripe for production. It kind of worked, but there would have been lots of work to be invested (probably more than Apple could afford) to turn this into a mainstream OS.
MkLinux was released in February 1996 whilst Copland got officially cancelled in August 1996.
So it's definitely conceivable that internally they were considering to just give up on the Copland microkernel and run it all on Linux. And maybe this was a legitimate third option to BeOS and NeXT that was never made public.