There are many ways people can contribute to open source outside of writing code. You can always do things like testing, documentation, demos, UX/UI design, project web sites, etc. The primary reasons that people don't contribute to an open source project are capability, time, and willingness to work on something for free. You really can't do so unless you have the ability to do so, which instantly filters out all the people in this little twitter discussion. Once you make it through that filter, you have the time constraint to work with. Many of the people who can do this work are busy with jobs. Finally, you might have the ability and the free time but simply want to do something else and not have to worry about fixing bugs and dealing with bullshit on nights and weekends. At the end, you are left with the current set of people who actively work on and make meaningful contributions to open source projects. Outside of that, you have a bunch of people who see the impact, value and societal impact of open source and want to be involved, but the only things they have to offer are idiotic ideology, politics and slacktivism thus you have people like this who try "fix" open source by flailing around, throwing tantrums and actually hindering progress.
Additionally, I don't see many cases in actual tech employment where a github profile is a major deciding factor. Sure, you have some high profile jobs at high profile companies where it might be meaningful because they are looking for the cream of the crop, but getting a tech gig at a bank or a big Fortune 500 enterprise -- which is where most of the tech jobs actually are -- doesn't require that. Most of these companies are getting by with people who barely speak broken English and show up to work in filthy clothes. The bar is pretty low, but the baseline is actually being able and willing to do the work consistently. If you can't do that, it doesn't matter how many "talks" you've given or how many followers you have on social media.
The reason John doesn't share any useful code on GitHub is because he is unable write any useful code.