The easyJet would be operating under Instrument Flight Rules (IFR) and most probably in controlled airspace.
3.6.2.1 Except as provided for in 3.6.2.2 and 3.6.2.4, an aircraft shall adhere to the current flight plan or the applicable
portion of a current flight plan submitted for a controlled flight unless a request for a change has been made and clearance
obtained from the appropriate air traffic control unit, or unless an emergency situation arises which necessitates immediate
action by the aircraft, in which event as soon as circumstances permit, after such emergency authority is exercised, the
appropriate air traffic services unit shall be notified of the action taken and that this action has been taken under emergency
authority.
ICAO Annex 2 — Rules of the Air
The crew would be required to seek clearance from the air traffic control unit before deviating from the established flight plan or previous clearance. (i.e. they would have been on a set course and set altitude).
Deviating without proper authorisation would not be standard procedure and could lead to potential safety risks or conflicts with other air traffic.
Furthermore, I’m not sure if this would have gone down very well with the easyJet operations team - increased fuel burn and potential to run late. However, the article does suggest that easyJet supported the move, though this could have just been a comment from the PR team.
(Also, I just wanted to highlight that decision-making in multi-pilot aviation involves both pilots, not solely the captain. While the captain serves as the final authority in making critical decisions, it is essential to recognise that the flight crew operates as a cohesive team. You make it sound like one person is calling all the shots here).