MEL vs MMEL
They may have operated outside of the provisions of the MEL, but, the MEL is a company document approved by the Authority.
The MMEL may allow operation without EPR indications if N1 indications etc are available to the flightcrew.
In that case all they had to do was request approval to operate to the MMEL limitations and not the MEL to recover the aircraft to base, or, submit an amendment for approval by the Authority.