Imo, the issue about the licensing thing is NOT that they were not credited, nor do they want to get credited.
The distinction is using GPL.
GPL is used when you want ALL derivative works to be open sourced as well. This means anyone that wants to make money (by selling software) CANNOT include your code
(unless they use a roundabout way of using your software e.g. server/client implementation ala MySQL).
Because anyone can take all the released open-source code, and recompile it on their own FOR FREE.
In other words, you use GPL when you DON'T want anyone to make money using your code.
If you just cared about getting credited, you could probably use a less restrictive license like LGPL or MIT (where commercial derivative works can reuse your code, and become closed source, and just leave your name somewhere in the credits).
TX had two options... either take the Atmosphere code as a base to sell their own homemade XCI loader, but they would have to violate the GPL's terms. Or they could write their own CFW base code, which would take forever, and they'd probably be releasing SX OS only now if not later.
I mean, I'm not saying it's right, but I see why they did it.