> Are we only learning about this because it's become recently declassified?

The only classified part of this story is the relatively uninteresting part - that this was to be a nuclear missile base. From https://en.wikipedia.org/wiki/Project_Iceworm :

> While Project Iceworm was secret, plans for Camp Century were discussed with and approved by Denmark, and the facility, including its nuclear power plant, was profiled in The Saturday Evening Post magazine in 1960.

That's a matter of perspective, apparently, to me that is the most interesting part. The fact that it took decades before it was declassified and only then caused an inquiry by the Danish parliament indicates that some other people thought it was interesting as well, at least interesting enough to attempt to keep it buried forever.

I think, perhaps, he meant "uninteresting" in the sense that the obvious assumption was that it would be a nuclear missile base, while the hope was that it was something much cooler.