SAP has the BTP and all their different products, so IaaS is very much the issue for them and I doubt they have solved it without the help of Microsoft.
StackIT is Germanys best shot at a sovereign cloud currently. Basically a fork of OpenStack.
I very much doubt SAP has the competence to replicate this. I'll also bet those unnamed open-source technologies are mostly sprinkled on top of proprietary Azure tech, like the rest of their infrastructure.
I've been deep into the OpenStack ecosystem and community from around 2011 to 2022 and I have to tell you that besides the steep learning curve it's a great system.