When WB did their restoration in 2000, I think they had access to whatever cut negative of the deleted scenes was needed for Richard Donner to insert the eight minutes that were edited back to the film, which is why the sections were presented in widescreen on the blu ray.
This will probably be my last post about this for a while (everything else I’ve had to say about STM RIC has already been said), except I’m sure more will be said when and if WB and/or Criterion makes any official announcement on a definitive edition of STM for the upcoming 40th anniversary. In the meantime I hope I have cleared some of the air on the state of the STM RIC.