Yep, it's true. We (
Team Caltech) got the official letter yesterday. We contacted the DARPA folks who said the site visits are meant to weed out the applicants who, for example, have no sponsors, no money, no technical advisors, or have made insignificant progress. Limiting the number of entries to teams deemed to be competitive makes some sense for several reasons, like minimizing the number of chase vehicles that DARPA has to provide.
-Kevin