2.0 is under a truly open source licence, and the 2.0 security releases were carried out under that restricted environment.
The licence does not affect the way in which the developers choose to develop the project, they are free to develop it entirely under wraps. I don't believe MyBB (LGPL) has their repo fully open to the world, for example.
I'm not a lawyer nor do I play one on TV so don't quote me on this but as far as I understand only 2.0 and forward are under an open source license, that is, any RC and 1.0 and 1.1 releases are still under the SMF license.
As for the development process, yes, the license doesn't force you to keep your environment open but, it is better to keep the "official" environment open than to let somebody else to upload the SMF code to bitbucket or whatever and to fool people to think that page is the official one.
We can't really stop people from doing that but at least we can be truly transparent about the "official" development and people can make their own conclusions on it.
Except that you get that in addition to the 'I downloaded it but it isn't ready yet' moans. And that still doesn't detract from the 'why isn't it ready yet' moan. To me, it makes more work, not less.
Yes I agree, there always will be people complaining about this, this will not change, it happen in the past and happens now and will happen on the future.
This doesn't have to do much with what license or develop environment we chose but has to do with the whole free thing, being free means a lot of people, different kind of people has access to this software, this will inevitably will attract people with, lests say, not so much understanding about software and stuff