The photos you attached aren't of the hedgerow in question, though the description should be sufficient. 'Green Giant' Arborvitae mature width references vary with what age of plant the source is referring to. One reason is because local growing conditions affect how quickly a plant will grow; another is because plants never really stop growing outright, so over time, old plants will keep getting larger, albeit at a slower pace as they age. Overall, these Arborvitae grow much more quickly in height than in width. Usually, 'Green Giant' is said to get about 6 to 8' wide, and this likely refers to a plant that's around one or more decades old. Ten or perhaps twelve feet in width is certainly possible given another decade or so.

When eventually crowded at their base, as with most conifers, they'll likely lose some foliage in places that are too shaded, but such gaps may not be apparent unless lower branches are lost at some point in the future because the outermost growth, which receives more sun, will be covering them up. It's more of an aesthetic issue than a health issue for the plants. Still, staggering them is a good way to still allow for a good view obstruction while giving the plants more breathing-room.

Miri