Are you using SketchUp Make? This seems to be directed towards people not using SU Pro (even though the text fails to say it out loud) "If you are learning SketchUp, creating 3D models, downloading an extension, OR developing an extension for your internal/personal use you DO NOT need a SketchUp Developer account". If you are planning to sell extensions for money that's probably commercial use that requires a Pro license. However I think having a Make license is a really bad reason for not being allowed to be a developer. I had never used SketchUp commercially and was still using SketchUp make when I became one of the top developers in Extension Warehouse. There's no reason why Make users shouldn't be able to share their extensions with others.