THis is worse than the Aizen crap from Bleach.
A good plot twist comes from the little clues and hints comming together to an unexpected truth. Not by retconning everything as "it was all a lie this is the real truth". IT's forced, it's lazy, and it dumb because the only reason we couldn't see this twist before hand is we were intentionally lied to.
At least with Aizen he was built up more as a manipulator who used illusions and using people as pawns. So while the "all according to keikaku" nonsense got old it wasn't entirely out of place. But here you might as well consider the old Merlin and the current Merlin two entirely different characters. While she'd been implied as a mastermind of sorts in the past it was nothing like this.
The author even threw in her being in love with Meliodas out of nowhere.... Tell me comment section. Did ANYONE want to see Merlin retconned into a love triangle with the main couple? AFTER what happened with Escanore?