The definition of what counts as a "derivative work" has nothing whatsoever to do with the AGPL: it's defined by copyright law. (Although you're right that it's

The definition of what counts as a "derivative work" has nothing whatsoever to do with the AGPL: it's defined by copyright law. (Although you're right that it's almost entirely defined by case law, rather than being explicitly defined in statutes or regulations.) If something is a derivative work for the purposes of the AGPL, then it's also a derivative work for the purposes of the GPL, or any other copyright license.