Sharing a fifth of revenues may be an industry standard in the app world, but what are you getting in exchange for this cost? Clay.io is providing tools for payment processing, persistent data storage, social integrations, and the ability to distribute your game to multiple marketplaces. It seems like a fair trade to me, at least initially. I suppose that if a game became a blockbuster hit, it would be worthwhile to reimplement functionality offered by the Clay.io API in order to recoup that 20% share.
The Clay.io team looks to be a small group of mainly college students. That puts them in the indie category in my book, but the members bring some legitimate experience to the table, and have also secured a collection of more experienced advisers. I wouldn't be surprised to see this group ride the wave of HTML5 game development and found a sound business in the process. It looks like they are in a busy API space with over 130 games APIs in our index.