"Airbus only" fleet in easyJet's case means Airbus A319-100, Airbus A320-200, Airbus A320neo, and Airbus A321neo.
A Boeing only fleet could have 737NG and 737MAX. This doesn't help if you are completely against flying in the MAX.
I think the point was that youre guaranteed to not fly in the MAX if you go with an airline with an airbus only fleet
Good point. But then you wouldn't really avoid "737MAX" you'd be avoiding "Boeing". If the price increase theory is correct then you'd end up paying a lot more since you restrict the options of planes a lot more.