I saw something in print about them last night and just googled "sergey larry montessori" today to find this page. There are other pages farther down the search results that seem better researched, but the basic idea is still that Montessori gave them freedom to explore interests.

One additional thing in another article is that Montessori let Sergey learn at his own pace, important when he was first learing English:

http://www.momentmag.com/Exclusive/2007/2007-02/200702-BrinF...

There's a 20-minute video here, with a synopsis that credits Montessori with inspiring the 20% time idea:

http://www.ted.com/index.php/talks/view/id/118

Many pages repeat the "friends since childhood" line for whatever reason.