It's fairly easy to obtain the weights. I've had two of my friends downloading these weights and sharing them with me, so it's probably not surprising that the weights got leaked.
Was every Meta employee able to download them or why did so many people have access?
Anyone can submit a request form and enter their email address to request access to the weights. People who have a .edu email and are involved in deep learning are likely to succeed and get a download link sent to their email.