I keep linking..?

An atypical example of something is still a “true” example of the thing, especially given that the very term derives its origin from Stonehenge itself.

I think the article is a bit confused in both defining the "henge" and saying if it is a henge or not. It sets a definition that Stonehenge doesn't follow, calls it "atypical", says it's not "true" henge (whatever that means) and so on. And all of those seem to be not directly sourced. One of the sources makes the confusion even worse:

Ironically, even though Stonehenge has an earthwork circle around it (the earliest phase of the monument), it isn’t officially a ‘proper’ henge, as the main ditch is external to the main bank. It has to make do with being a ‘proto-henge’.

I guess in this use it a "proper" ("true"?) henge is a henge and "proto-henge" is not a henge but a thing that precedes them. That'd explain some of the confusion. Could also be that "proto-henge" is counted as a form of a henge, but I'm not sure. Other examples seem to consider it preceding actual henges.

It would help if there was one definition for henge but there seems to be some that define it like in the article, with the ditch inside and others that just include a ditch.

especially given that the very term derives its origin from Stonehenge itself.

Terms can change over time, it could be the definiton of "henge" has changed.