It is very unlikely that any airline would lie to you about the reason for a delay/cancellation. Even the LCCs. The National Enforcement Bodies (NEBs), media, claims brokers and lawyers would very soon catch on to the attempted deception and would likely result in enforcement actions. (depending of course on the national law, 32 versions currently in play. EU28 + EEA3 +CH).
If 'flow management' results in a 10% reduction in capacity this means that 90% of flights will likely operate as planned.
Try one of the claims brokers (a short search away). They have the means to check U2's claims and to judge the likelihood of success, for a fee ...