This article is incorrect. If you read the actual FCC documentation it explicitly says that 3rd party firmware modification is allowed and in fact, encouraged. All that is required is for the original equipment manufacturer to have an agreement with the supplier of the 3rd party firmware that the 3rd party firmware maker will not permit operation outside of the authorization for operation in the United States.
All that TP-link has to do is have dd-wrt sign a document saying they won't put in functions allowing users to make RF changes, that is all the FCC requires. Then TP-Link can issue a boot security key that is compiled into the dd-wrt firmware by the dd-wrt project that will allow dd-wrt to boot on the TP-Link routers.
The fact is that TP-Link just doesn't want dd-wrt on their stuff for whatever reason, and is using the FCC as an excuse. I don't see the problem here users can just buy gear from router vendors like Linksys who work with dd-wrt and are friendly to it. There's no reason to bother with TP-Link's stuff.