That's what makes them look incompetent. They insisted that they couldn't do it without Apple's help. Turns out they could, and they just didn't do their homework first. (Or, if you believe they had ulterior motives, then they didn't want to.)
Those are two radically different values of "know everything" you're pretending are in conflict, there.
Yes, I expect the FBI to "know everything" when it comes to techniques available to do their job. No, I don't expect the FBI to "know everything" when it comes to the contents of my private messages. There's no conflict.
