The whole E+ marketing / branding is interesing. Its a cabin within a cabin. Its similar to middle seat blocking, in that its not guaranteed.

I'm not even sure UA has quite decided if its a separate cabin, or the same cabin. Y/B seats can still be sold if E+ is sold out. There are no fare classes associated with seats in E+ (only you can sit there if there is space). Inventory management does not manage E+ inventory. FAs may/may not enforce seat assignments in E/E+. etc etc.

While there are advantages of this, (not least of which is the employer that only pays for the lowest class and would not therefore pay for premium economy as a separate class ), the whole positioning of E+ is somewhat schizophrenic IMO.

So is it an op-up. Its in the eye of the beholder. If you believe you have been op-uped, and it makes you happy, then go with it.