I certainly don't think the SPLC is omniscient. They have made mistakes in the past, and will make mistakes in the future -- exactly like everyone else. And it's entirely possible that the claims in the story you linked to are correct and this is another example of a mistake (although the article offers no actual evidence that this is the case).

However, none of that means that the SPLC is wrong when it comes to the FRC. It has been obvious to me personally that the FRC is a hate group from the very first things that I heard them say and saw them do.