Those words, in both the case of the DCO and GPL, are self-referential; you cannot modify the DCO or the GPL freely. You can modify the work the GPL covers, and the contribution in the DCO is generally licensed under the license of the project you're contributing to.
In that sense, the DCO itself and GPL itself are technically proprietary documents. This isn't a problem in practice, you should not modify these legal documents at home, that's a bad idea, and it's not a super important freedom to preserve. But if there's a technical concern distributing those documents with Debian because those clauses cause issues under the DFSG, that's something you'd have to figure out.