some nothing club that nobody has ever heard of.
Congrats, you've just successfully identified yourself as "nobody". (Slovan is a former European cup winner, having defeated Barcelona in the final.)
The average attendance in that Slovak league seems to be <3k per game, so most likely this teams entire fanbase was able to get tickets and leave a bunch left over for Man City fans.
Why do you feel the urge to talk about something you have no clue about? Nobody is interested in Slovak league, but European games regularly sell out. (And back in the days of Czecho-Slovak league, sellouts were common, too. The situation is identical for hockey and football.)