The sane bootstrap is not about porting to new architecture, but about building the compiler from source in the first place. CMU CL effectively requires that you use the same version of CMU CL to build it, IIRC, which in practice meant you had to first load the changes into older CMU CL image - overwriting core components, and then use that image to compile new version.

The difference with SBCL is that it can compile so long as you have any CL implementation that handles certain broad, but not complete, subset of ANSI CL.

It does it by being able to compile itself in sort-of sandboxed state hosted within another CL image, and then using that compiler to build itself, without requiring to modify the host compiler like CMU

As for the bytecode, as far as I recall CMU CL did not have bytecode compiler at all - it had evaluator option, which was indeed dropped for a long time from SBCL but was recently reintroduced, partially to support efforts such as porting Kandria [1] to Nintendo Switch

[1] https://kandria.com/