Inline 6 inlet manifolds on engines with three siamesed inlet ports are connected as you describe, with results understandably ranging from woeful to simply horrid. But the headers in the pics when on the intended engine (and likely every other inline 6) are connected in pairs to cyls exactly 360 deg apart, all three pairs.
Edit -> Yeah, sorry panic, I took another look at those pics and it does look a bit sus. The last ones I saw in the flesh many years ago were as I described, but these ones maybe not. I’ll have to look a bit further into this!
Edit 2 -> They are plumbed correctly. They are grouped 1 and 6, 2 and 5, 3 and 4. They way they are layered is different in the two pics. I got confused about it too.