I think it's one of these "reading the letter of the law" instances. European laws (or rather, laws in European countries) often mandate public sector to use open source. The reasons vary, some of them are about promoting interoperability, and avoiding vendor lock-in, digital sovereignty, and the EU commission has a principle of "public money = public code".

So using open source on someone else's computer technically fulfills that requirement, without completing some of the reasons why the requirement exist (vendor lock-in in this particular instance is particularly laughable).