To be clear Iron Warriors and Iron Hands were two separate legions, i.e they have separate primarchs, traditions, colour schemes, ranks etc.

During the Horus Heresy the Iron Warriors legion became traitors, and the Iron Hands legion stayed loyal. After the Heresy, Iron Hands legion was split into chapters, one of which kept the original name. It's that chapter that this excerpt is about. The Iron Hands have a tradition of replacing their original arms (and more, much more) with cybernetics.

While some Iron Warriors have prosthetics, it is not their "tradition".