Luca is mostly used for contact tracing (I did not even knew you could store certificates in it) so the Allianz Arena meant that they will use another form of contact tracing (paper, a QR code to scan that opens a form in a browser, another app).
The vaccination certificates can be presented any way you want. And mlin32 is right, absolutely nobody is scanning them in Germany, which seems to bother no one weirdly enough.