It’s not really proprietary. The code is still open. They’ve just locked it behind a paywall. There is the developer subscription that anyone can get 1 or a few of, can’t remember, so you still have access to it. Legally, based on the GPL, they HAVE to give access to their customers.

They also do contribute back upstreams.

But there are somethings they basically put together behind the scenes that would be very difficult for you to do yourself.