Did BRG have that concept patented?
Doesn't matter (but would help their case if it were). Note that the lawsuit isn't for infringement (patent or copyright) but for breach of contract and theft of trade secrets (that Facebook allegedly only had access to in confidence, i.e. via aforementioned contract). It all depends on if Facebook's agents signed anything similar to a NDA when negotiating with BRG for a design contract, in order to review a proposal using their "modular techniques". If BRG was smart they would have papered it up very specifically before they showed any sensitive bits to Facebook.
Like TFS says we don't have enough info to know if something super specific about the design was copied (like some allegedly optimal ratio of airflow to floorspace to TDP). This is most likely just a contract chase, hoping that the words of whatever Facebook signed are broad enough to catch them for designing anything similar to what BRG had proposed.