Arc is most definitely "open source", not "proprietary, source available". It's provided under the Artistic License (https://en.wikipedia.org/wiki/Artistic_License). That license is GPL compatible, upholds the "spirit" of FOSS, and is OSI certified.

A bazaar development model is one that takes place in the open with contributions by strangers actively welcomed. A cathedral model involves development behind closed doors with periodic releases. (https://en.wikipedia.org/wiki/The_Cathedral_and_the_Bazaar)