Theory
What if Nora made Thomas Anders date Melanie Sanders, as a publicity thing, but eventually she became jellous and decided to get rid of Melanie, and Melanie didn't really like this so she made the song titled "Ich mach Schluss mit Tommy", which translates to "I'm breaking up with Tommy", Nora probably didn't like this so she silenced Melanie to not reveal anything, which is why there is so few things about her and Thomas
Nobody knew who Melanie (real name Ingrid) was in the 1980s. I mean she was not a star and not that famous to be used for publicity. In addition, nobody even knows that those photos (where Thomas and Ingrid pose together) exist. But it can be possible that they dated in secret.
I think he meant "in public"
I think the Russians found these photos in magazines, so it makes sense that they appeared in some of them as a couple, so that no one would think it strange that Nora was Thomas' girlfriend.
Frankly nobody knows how they got in possession of such pictures. Or where exactly they came from. It's much more likely though they got them from documentaries which most likely aired more than 20 years ago (the age of recording TV shows or music videos with DVD recorder/cassette). Thanks to the antennas it was possible. The pictures don't look like they came from magazines. There is no texture of paper in them. So they most likely got them from documentaries.