If you were told it was ATC then it was very likely ATC that caused the issues. Thunderstorms are very dynamic in their nature and they can cause severe flow restrictions at some stations which lead to slots so even though the skies are gin clear, departing or arriving may be hampered by slots. There's never a conspiracy between crews/OCC and the airport ground staff to falsify the reason for a delay.
I've never once lied to a passenger about the reasoning for a delay, I've never been asked to lie and if asked to lie I would certainly refuse to do so. The app is always the most up to to date source of information.
Either way there out of the airlines reasonable control and therefore you won't be entitled to compensation as with every other airline (nobody will give you money back unless they need to although you may get FF points if your airline has such a program and you're a member).
Keep I'm mind also that as with all airlines U2 jets operate many rotations throughout the day. Looking at arrivals/departures, your aircraft operated the NCE (all early) then delays picked up in BCN and picked up the delay leading to knock on delays whereas other aircraft may not have been through BCN and would not have picked up the delay.