I really enjoy Digital Foundry. I've been following their work since the first quarterly comparison article Richard started writing for multiplatform games between the Xbox 360 and the PS3, circa mid 2000s. The thing to remember is that all of this is human observations, backed up with math where applicable. You're analyzing what you are capturing. I'm not sure of all the steps necessary to do so, but counting and eyeballing the images trying to find differences are part of it.
Digital Foundry are experts at doing this, which is no surprise since they've been doing this for more than 15 years. Even so, there have been times where they were wrong. Well, not necessarily wrong in the sense that the data they show is wrong. But more like they sometimes miss some stuff. Or, perhaps, what they are suggesting do not always correspond to what we're viewing in their comparisons.
Like it or hate, there is a case to be made with this picture:
The lack of trees and certain geometries in the Series S picture compared to the Series X prove that the LOD value between these 2 consoles is quite different. On the other hand, you can clearly see that the One X picture does not have nearly as much trees on the foreground or geometries on the mountain in the background versus the Series S version. On this picture, it's hard to argue against the following: Best LOD value is on Series X, followed by Series S and then One X in the last position. Whether Digital Foundry says otherwise is a different matter, on the scenario shown on this picture comparison, you couldn't convince me otherwise just because DF says so. Remember, they're doing their best to paint the portrait and show us the results, but whatever verdict they reach isn't to be considerate as absolute. I distinctively remember one scenario where they were comparing Rise of the Tomb Raider between One X / PC and PS4 Pro and they were showing that scene where Lara is about to grab an artifact in an ancient temple and they were saying that in that screenshot, the PS4 Pro version clearly had less detailed textures, but further testing from GAF community proved that they were wrong and that what we were seeing was the results of dynamic dirt effects on Lara's clothes.
It happens. Doesn't remove their credibility. It's the nature of such analysis. You can't really give an absolute answer unless the developers themselves are stating pure graphical facts.