The weights is the source code. You are looking for design docs or something.

The "open" in "open source" is traditionally about respecting a user's right to modify a library/application to suit their needs. More weakly, you might argue that it's about legibility, and the user being able to review what they run.

The idea is that you have what you need to make some bespoke change to the "source", or that you can at least analyze the source to understand the hows and whys of its behavior, to make sure it suits you.

Do weights provide either of those qualities?

That’s not true at all. The weights are the outputs of training. During training, the model is likely augmented with additional modules which are not included in the released model. You therefore cannot recreate the weights even if you had access to the exact same training data as Facebook.