"I get that if the code is their copyright, they can dual license at will. But doesn't the above mean any contributions from either a community or "Membership" cannot themselves be GPL, since any code accepted will in turn be distributed "non-gpl" among the membership? Also, are there "multiple tiers" of "non-gpl limited license"?"
I think you misunderstood. If you obtain a copy of the source code that is licensed and distributed as GPLv2, as they claim to make available, and you then make a patch for that code then your patch must also be available under GPLv2. Otherwise there would be a license violation.
On the other hand, if you buy support, they will give you the source code under a non-gpl license which they have every right to do since they own the copyright for the original source. This can not contain any GPLv2 only code (unless under the original GPLv2 license), say from contributors that got the code under GPL. This is mainly for companies wishing to make changes to the code without having to release their changes under the GPL.
That said, requiring a 'support contract' so that they provide you with the code under a different license is pretty low.