The Minecraft forums thread specifies that people should not distribute modified versions of the mod. However, as this mod is under GPL3 and that license specifically allows for modification and distribution of the modifications, in both source and binary form.
I have to admit that I didn't look everywhere, but in the few places I looked, there were no additional clauses that modify the license in the source file headers and tbh, having the mod under GPL and disallow distribution is weird, as GPL is all about forced sharing.
If there are only a few contributors, you might be able to change the license without much hassle or add a clause that allows modification and redistribution only as reuse of parts in other mods but disallow redistribution when used as a major basis for a mod.

Whatever, I just wanted to point this out. I've no vested interest in the mod changing licenses or staying the same, nor have I even tried the mod. I just saw this weird conflict while reading about it and wanted to raise the issue.
This looks like quite an interesting mod and I'll be looking into it a bit more and I just love being able to read the source, if only to educate myself.

I am aware of the license conflicts, yes. The permissions are mainly there to scare off those who may try and redistribute it on other hosts. Terrible, I know, but hey, it worked. MCA will be moving to another license soon that makes this more clear.