Wasn't AmigaOS built on other systems most of its lifespan anyways? 
I think it's closer to 55:45 given the whole time span between 1983-1994 (assuming that the operating system took shape around 1983 and not 1984).
At Commodore they used Sun 2 and Sun 3 machines for development, but I don't remember what was in use at Hi Toro/Amiga: there was a reason why the Sun workstations were preferred, and it may have had something to do with how expensive they were.
Native Amiga development tools continued to improve through 1987/1988, and work on what became Kickstart 2.0 did use the native Lattice 'C' compiler, several versions of which were used during the development of the subsequent 2.04, 2.1, 3.0 and 3.1 operating system versions. The CDTV project most prominently used the Aztec 'C' compiler.
It could have been Commodore's penny pinching which drove the developers to native tools, but the improved performance of Amiga hardware, along with the better quality of the native development tools did yield better quality software. A whole battery of tools for QA work was created specifically for Kickstart/Workbench 2.0 which the original operating system developers could not use (due to lack of MMUs, for example).
One downside of this move from cross-development to native development was that the ability to build the entire operating system on a single machine was lost. The local builds (still using makefiles) drifted apart, and by 1994 you could expect that no two operating system components were built similarly. In fact, you probably had to wrangle three different compilers, two different assemblers and start the respective build manually. While it was still used in production work (1989-1994) that "build process" must have produced and lead to the resolution of integration problems on a major scale
What I find ironic here is that ThoR uses Linux, of all things, to get work done.
Actually, one of the reasons why this approach was picked was the idea of having a build server which at the end of the day would crank out a complete AmigaOS build, or stop and complain if this didn't work out ("the daily build & smoke test"). We couldn't conveniently do this with an Amiga, but with the vamos setup it became possible.
I'm assuming you've done your lot software development work. One of the most important aspects of software development productivity is to be able to build and rebuild the product as quickly as possible. If you have to wait hours just to find an integration error, chances are that the whole development process will make you utterly miserable.
Eating your own dogfood and everything which building the Amiga operating system on an Amiga, and testing it in this context brings with it, there's no single tool which can do everything well. I'm accepting that a build done on any POSIX system through vamos is now part of the process of building the 68k Amiga operating system. Irony be damned
