Presumably they mean non-Bazel external projects, which that link doesn't even touch on ("The other repository uses make... just rewrite their build system!") - an external project with the same build system is the "easy" case for things like this and CMake; any new contender has to accept that there is a wide ecosystem of other solutions it's going to have to interoperate with.

...unless you are in a corporate environment with a build team to integrate complicated external projects into a monorepo and where things like build system can be mandated.

That's always the feeling I've gotten from Bazel - you either do things the "google" way, or find something else.