This is an awfully big detail to just get wrong. The D7 is almost as iconic as the original Enterprise. This was deliberate, and a clue. I think we're getting pieces of a jigsaw puzzle and it's starting to make a picture:
- We know there's not just one 'Mirror' episode coming, but several.
- Spore drive appears to be successful. Even the ethical problems have been solved. But that just can't be since there's nothing like it that is ever used again in Star Trek.
- Everything that appears out of whack will be explained by the time the season ends.
- Higher-ups have decreed 'no round nacelles' on Starfleet vessels.
- Uniforms that look like the original Cage attire were made and seen by visitors as hanging on racks, but aren't being used.
- Why even keep those uniforms out where visitors can see them?!
- Lorca is the only survivor from the U.S.S. Buron.