I think there is no way to download all these maps at once, and Firefly hasn't included these maps into any of their Stronghold releases. That is, as far as I know.
Simply, this list has been created by fans from this community, and not by Firefly. Though I don't understand why haven't they included these maps into some of their releases.
However, Firefly did include many fan-made maps into their Steam version of Stronghold HD, but I feel it's not even close to 300 maps.