The article says that multi arch makes cross compilation much easier, you get it "for free".
This is wrong.
It probably helps with RPATH, but that's about it.
Cross compilation means that the configure-process (autotools, cmake, scons, whatever) must not use properties from the system it runs on to conclude how the target system looks like.
E.g. it still cannot run executables on the host system for the target system (try_run() in cmake). The buildsystem must still know that it is cross compiling, and handle try-run tests accordingly.
It must still be aware whether the executables it finds are for the host or for the target, and whether it can actually execute them or not.
Really, the one thing where I see it helping with cross compiling is RPATH.