Because that's exactly what he says?

It also follows ordinary narrative convention more closely. We can tell from distant shots of the Death Star that there’s an equatorial trench, but not other trenches. If you’ve already introduced a trench, conventionally you’d just use that if you need a trench. The difference between a narrative and just a bunch of stuff happening is precisely that kind of re-use and tying-together of elements.

Idk, they do clearly fly into the equatorial trench the first time we see the Death Star, and it is clearly very, very wide.

I never paid attention to the briefing graphic which showed it was a longitudinal trench, so in my head canon it was always lateral. But not the equatorial trench. I'm also a bit dumbfounded that misunderstanding is/was so common.