It' still also quite likely that they will deem the diffusers illegal but only punish McLaren about this:
See, they (the FIA) know McLaren believe the double decker diffusers to be illegal but they have not protested against it.
Thus, a competing Team deliberately allowed 3 other Teams to race an apparently illegal part without bringing it to the attention of the FIA as they were obliged to, according to Sporting Regulation No ...
(apologize for separating one subject to three different paragraphs against everything I've known since 1st grade, but this is only to satisfy popular demand)
The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. H.P.Lovecraft