There's no logic in assuming that every visual ever seen in Star Trek must be taken literally, because a great deal of it is physically impossible (visible beams or roiling fireballs in vacuum, brightly lit ships in deep space, ships crawling past each other at mere meters per second when the story says they're at high impulse), contains errors or in-jokes, or recycles stock footage. A lot of Delta Quadrant starships in Voyager were reuses of Alpha/Beta/Gamma quadrant ships in other series, even though trade between the respective civilizations was impossible. It shouldn't be taken literally, any more than we should literally believe that all the characters played by Majel Barrett or Vaughn Armstrong really looked and sounded exactly alike, or that Saavik or Tora Ziyal underwent a complete transformation of face and voice and nobody noticed. Or, for that matter, that Captain Kirk, his crew, and the entire universe around them suddenly transformed into cartoons for about a year and then turned back to live action. It's at times like those that we need to move past rigid literalism and accept that what we see onscreen is merely a dramatic representation of a hypothetical reality, one that sometimes uses approximations, substitutions, and symbolic representations. We're supposed to use our own imaginations and intelligence to look beyond the surface and visualize the "true" reality that the images are just approximating. We're not supposed to take every tiny detail as absolute literal truth that must not be questioned.
So no, that was not a D7 in that Enterprise episode, not in-story. It was a stock image that the creators of a fictional television program used as a substitute for an earlier Klingon ship as a convenience, and that they replaced with a more period-appropriate design at the earliest opportunity. It was, essentially, a mistake, one that those involved with the show admitted and corrected.
Same with the Mirandas and such in TNG. They used those because it was more economical and practical to reuse pre-existing starship models from ILM than to spend money building new ones. Sometimes they used them even when it was inappropriate, like for the Lantree in "Unnatural Selection," which was clearly intended in the script to be a far smaller type of ship. And then there's the use of the same Klingon Bird of Prey miniature to represent two vastly different sizes of Klingon ship. I just choose not to take those literally. What we actually see isn't some divine gospel we have to accept without question; it was a compromise made for the sake of economy and expediency, reusing a design that didn't make sense in context because it was the closest substitute they could afford for the intended "reality" of the story. And that's where it's incumbent upon us as viewers to exert our own imagination and critical thought instead of just slavishly accepting every last detail, even the mistakes and compromises, as unquestioned fact.
