So, is this fake? https://www.stackit.de/en/cloud/products-services/

no, definitly not. but Lidl don‘t run a single production service on it yet.

I guess you have inside information but my naive assumption, reading the website doc and guessing how this things work, is that Scwarz group created it on top of their already existing self-service infrastructure, they "just" opened to external customers. Or did they create it from scratch?

where do you got that 8000 developer number from? Sounds like a lot

You have misread, it talks about the open source platform OpenStack, and that over 8000 thousand developers work on OpenStack.