A shift to open source is sanctioned from the very top. For example, in the area of OZG (broad digital access to public services, coming soon) the explicit decision is (my translation):

> Open standards must be used in the implementation and operation of digital offerings. The source code from the realization of digital offerings by the administration (in-house development) is made available as open source, i.e., in reusable form, wherever possible. https://www.it-planungsrat.de/fileadmin/beschluesse/2020/Bes...

They are also starting a project to host public, open source code which is backed by the federal CIO and many state and local governments: https://www.cio.bund.de/SharedDocs/Kurzmeldungen/DE/2021/pm_...

This is not something sneaked in by some techies. Every decision maker is probably aware by now what "open source" means.