I would actually write back to Easyjet and ask them to show you proof. Make sure you can prove the delivery of this message.
This is very important!
Because if this goes to MCOL or court, Easyjet has to pay all legal fees, even if they win the case.

ATC restrictions are not always extraordinary, esp. if this occurs regularly or if the caused the delay was very minor.
Airlines have to built in some buffer into their schedules, if they operate out of congested airports.