Ah, yes, I see what you mean now. Though I doubt any Bulgarian would put that on a tourist T-shirt. We don't have that many tourist designs (you've probably seen them all in the underpasses near the Presidency and Sheraton) especially ones specific to Sofia. If we made more, there'd be other stuff to put on them, so basically - Constantine isn't good enough for a T-shirt :D
And I get why the fact-tossing would seem odd to you. I've just grown accustomed to it. Maybe I can explain. That "don't dismiss us" thing you mentioned above is shared by Bulgarians as a whole, which you may have noticed. Because we were kinda dismissed historically and culturally during Turkish and Communist times. Now there's this urge to bring that history and culture to the attention of everyone, be it other Buglarians or the big wide world out there. Because we are hearing all the stories of Roman conquests, Arthur's knights, the fierce vikings, Pearl Harbour and D-Day, cowboys and indians, ninja and samurai and other things all over the globe, that are part of popular knowledge. And it came kind of natural to want to say "Hey, that's awesome, but look, we have some cool stuff too. Not as cool as yours, but still." And humble as I am, sometimes... sometimes it really is as cool. The less organised form of this is blurting out facts like this, usually relating with something more popular for comparison (i.e. that Eiffel Tower thing). It's a sort of cherry-picking to get someone to sit and eat some more of the cake. I don't really know if it's working, and I imagine it can get pretty annoying at some point. But this way of "quick advertising" is getting more and more popular, because stuff gets destroyed or forgotten. (Here's me doing it :P) Like the hotel built on top of the largest Roman amphitheater on the Balkans or the wonderful wooden house in the park, which is now a naked ruin, as the ornaments from there now grace a "palace" at the seaside.
Considering the script is written by a Bulgarian, such mentions are to be expected.