They use both. https://code.overheid.nl/MinBZK?q=dawo&sort=alphabetically as the official source, and https://codeberg.org/DAWO for the community parts.

We can insist everyone self-hosts everything, but is it worth it? Do you have your own email? It's quite normal for a government agency to contract a third party to supply e.g. furniture, instead of chopping its own wood.

clearly shows the need for federated forges, where you get to decide on your own rules but you still benefit from interacting with a broader set of collaborators.