Canon is the body of work, mistakes and all.
Mistakes can, and will, be ignored.
Like the "200 years ago" bit in DS9's "Doctor Bashir, I Presume". That would have put Khan and his ilk after the founding of the Federation, which is obviously impossible. The writer of that episode flat-out admitted that he screwed up. So, as an obvious mistake, that line can safely be ignored.
Same story here.
Remember, the only reason that weird-ass Klingon ship was ever called "D7" is because some studio bigwig jumped the gun and ordered the line put in, in defiance of every bit of actual work being done on the show. It was never supposed to be called D7, it was always supposed to be Sech class. And now that we know what the D7 really looks like (they obviously can't both be one), then the earlier bit can be ignored.
See how easy that was? Hell, I'm tempted to go to work on the Memory Alpha page right now.

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