“My understanding from investigations at the time was that CIA did hide info about terrorists infiltrating into the US from the FBI to protect “sources and methods.” “
That is both true and misleading at the same time.
Dig deeper.
There comes a point where systematic arrogance and incompetence becomes indistinguishable from deliberate malfeasance. Since the former is a far more frequent occurrence than the latter, it is more probable in this case as in many others.