This is a common misconception relating to all US carriers. None have a premium economy cabin. AA has MCE, DL has EC and UA has E+. All are steerage with a bit more legroom and perhaps some free booze. But, the seats are steerage, the soft product is steerage and they are sold as steerage with a slight premium for their assignment.

Bottom line is that you can't buy an MCE ticket. It's Y and you can then select or purchase a MCE seat depending on status and the like.