The D5 wasnt... no, you are correct.
However, the term D7 was never canon either, but is treated as such.
The question then of what is canon and what isnt is, I'm sure, an arguable question and strictly speaking, whatever appears officially in the TV series or movies is canon (even with all of its inconsistencies). A certain degree of fan-based non-canon material has worked its way into what is now canon, but even still, I wasnt actually saying calling the Enterprise D5 an inconsistency. However, for me personally, I've come to think of a D5 as the Light Cruiser created by ADB, and there is a wealth of non-canon material which the fan base has gone to great lengths to ensure not only is it consistent with canon material but also with itself as well. This adds much more depth, realism, believability and cohesion to the entire Star Trek Universe and I for one welcome it. It would be nice if Paramount acknowledged the efforts of its fan base and went along with that as well, instead of creating not just inconsistencies with non-canon material, which is neither here nor there, really, but inconsistencies with its own material.