The model is not "open source", but it is an open weights model.

You can download it from the link given here at the top and you can run it on your own hardware, with whichever open-source harness you prefer, without having to worry about token cost or about subscription limits or about any future degradation in performance that you cannot control.

The recent history has demonstrated that such risks are very significant.

Being open weights is important for anyone who wants to use an LLM. Being open source is important only for a subset of those, who have the will, the knowledge and the means to train a model from its training data.

Having access to the training data used by a model would be very nice, but the reality is that for a normal LLM user it is very beneficial to use an open-weights model with an open-source harness, but it would be much harder to exploit the advantage of having access to all the information about how the LLM has been created.