Thanks for the clarification. The inconsistency of those two statements still makes me nervous, though, and – as you point out – a CC licence (“Some Rights Reserved”) would have been the way to go.

The History Matters partners own the copyright of any materials that you submit

The terms and conditions implies that authors hand over a perpetual non-exclusive license for the work:

a perpetual, royalty-free, non-exclusive, sub-licensable right and license

This is a subtle but important difference, in that I would still own the copyright to what I wrote.

That said they should still be publishing this material under a Creative Commons license as – by its own remit – it is precisely the kind of material which should be in the public domain.