Re: "One of the largest neolithic sites"
Firstly: you'd expect "one of the largest neolithic sites" to be one of the easiest to find. Stonehenge and the surrounding barrows, ditches, walls etc are easy to find. And large. So the claim seems reasonable.
Secondly: The parts of the North Sea that were once dry land, e.g. Doggerland, were flooded recently by geological standards but too early for complex structures of the same kind as Stonehenge: Doggerland was flooded 8000 years ago, Stonehenge was built only 4000 years ago (very approximately).
Thirdly: They don't claim it is "the largest", they claim it is "one of the largest". Which it is.