It means you download a binary that contains support for every architecture, and then strip and discard all the architectures that you don't want? It's also more complicated than that - imagine a binary that drops some data in /usr/lib/whatever. If that data is architecture specific, you still need to go to the effort of modifying the code and packaging so your fatelf executable gets the correct file. And once you're at the point of doing that, you might as well just finish the job and end up with multiarch.
Fatelf and Multiarch solve different problems. Fatelf lets you distribute a single binary that works on multiple architectures. Multiarch lets you install binaries from multiple architectures on a single system.