The article doesn't suggest that the 10x API is going to be paid-only. It suggests that the firehouse is. Which sounds right, the relevant snippet -

"We want everyone to have the opportunity -- terms will vary based on a number of variables but we want a two-person startup in a garage to have the same opportunity to build great things with the full feed that someone with a billion dollar market cap does."

http://groups.google.com/group/twitter-development-talk/brow...