The centerline is not labeled, so it doesn't have to mean centerline of ship; it could simply mean centerline of bridge.Ah yes, here.
Then it all matches and makes sense: It is at an intermediate angle.
As @BK613 pointed out upthread, there's more than a passing resemblance between the bridge figure and the rendering of the bridge within the STAGE 9 diagram also reproduced in The Making of Star Trek. In particular, both have the same unlabeled centerline. There is no position of the ship relative to stage 9, so interpreting the centerline on the bridge in that figure as indicative of its orientation in relation to the ship would be absurd. Ergo, the premise that it's relevant to just the bridge set itself is more than tenable. And, being the case there, why not likewise in the other figure? It's unlabeled, after all.
So, freed of circular reasoning, the centerline on the bridge figure proves nothing. A label for it could speak volumes, but it's not there.
P.S. There are multiple centerlines in the STAGE 9 figure, three at least overall that I can see, and they sure aren't parallel. There's also the arc down the main corridor indicating its center.
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