I'm more inclined to think Piquet is stupid enough to have made up his side of the story than to think Briatore is stupid enough to have instructed Piquet to rig the race.
Briatore, for all his faults, is shrewd enough not to expose himself to such an obvious blackmailing opportunity. Piquet Junior, on the other hand, has shown plenty of times that he is prone to speaking without adequate employment of his higher-order thinking abilities.
Having said that, if it turns out Briatore and Symonds were privy to Piquet's deliberate crashing (if there exists enough evidence to prove the crash was deliberate, rather than a mere error), their reputations will be tarnished. This is particularly the case for Symonds, who is widely regarded to be as an honest character among F1 management.