AFL cannot fuzz ruby code using the same mechanism, because it does not have the right level of insight. AFL can't detect if a branch in my ruby code was tested, only that the code for `if` in the core ruby interpreter has been tested both ways. For this reason, in order to properly test interpreted code, you need to add the AFL instrumentation at the ruby layer, not the native layer.
I don't think AFL currently supports this, and I don't think this project actually enables it. But I would be happy to be corrected!