Rule #1 of getting paid for your work is don't do a bunch of work without signing a contract first. Even if you're an artist. I highly doubt he could have released that poem on his own and turned it into millions of dollars or whatever he thinks a "fair" contribution is.
Contributing an asset doesn't give you any entitlement to share in the company. If you want some equity in the company you better ask for it up front, because they probably won't give it to you and would just hire someone else and you can save your hard work. 20k seems fair and helped the author out a lot at the time. Posting a long rant about it probably hurts his future work opportunities.