It’s not open source, but is open weight - like distributing a precompiled executable. In particular what makes it open weights rather than just weights available is that it is licensed using an OSI approved license (MIT) rather than a restricted proprietary license.

I really wish these companies would release the training source, evaluation suites, and code used to curate/filter training data (since safety efforts can lead to biases). Ideally they would also share the training data but that may not be fully possible due to licensing.