JRuby, at least in my experience, has not yet delivered performance in line with my expectations of a JVM-hosted language.
In my opinion, performance is by a huge margin the chief reason I do not begin new projects on Ruby. I would rather defer performance tuning efforts and scale complexity until much later in projects' life, rather than immediately following launch when I have 100 concurrent users hitting the application.
However, the author does point to problems in Ruby's performance and maturity that are problems and issues -- if not immediately, then down the road. Your point about how people are leaving Ruby in favor of people, tools, and performance in the Java world is a good one. I would like to think that JRuby is a good option for such people, but maybe not.
I found the author's ideas for modernizing Ruby to be interesting. I didn't agree with all of them, but at least he's trying to hash out some possible ways in which Ruby could improve. This sounds like a wish list for Ruby 3.0, though -- interesting ideas that would break compatibility, and that will take a while to implement. Perhaps his work, and that of Rubinius, could show us the way forward to such a new version.