He basically said he wanted to start N number of startups under the umbrella of Archimedes Labs. This meant that when Arrington accepted to become "co-founder" as minority shareholder, he implicitly accepted that whatever he does can be accounted as building new startup under Archimedes Labs. Keith then tries to get credit of all work done by him even though he had no contribution whatsoever. The difference is that Arrington can threaten legal action unlike usual employees. However things could have easily gone other way around. Arrington might have yielded in face of litigation and giving Keith 50% share, making him bonafide co-founder.

One of the tactics people like Keith exploit is this: if you ever talked to him about what you are doing, his responses would be counted as "contribution" - even if they are complete nuance. For example, if Keith would have said "I don't think this would be successful, but do it anyway" then it gets counted as participating in vision exercise.