>> Also open source government code means other governments can fork it, overall lowering implementation costs, while still keeping code sovereignty. > This is completely unrelated.

This is an option which does sometimes happen. And there is motivation to make happen more often, at least for EU-wide services. And there is also the side that it's doesn't have to happen between countries, it could be also happen the local level, like between administration of cities in the same country. The main reasoning here is more about spreading awarness and building the mindset that sharing code on all levels and working together even on such internal tools, can be good and should be increased.

> French government won't deploy a Polish public health management website just because they found it on Github.

Some governments have also their own platforms, specifically for co-working on code accross administrations. They are usually not public for reasons.

> For projects of such magnitude you need deep mutual cooperation between both governments, and a lot of changes. Making the code open-source is the least important part, the code can be just shared privately.

You still have to put it under a licence when you are co-working, even when it's shared privatly. Open Source does not neccessaly mean that the source is automatically accessable to the whole world.