You don't mis anything.Indeed. The most extreme case of surpassing the 8 district figure would be in the case that the Magician has been swapped out for the Wizard and the Architect is kept in. Wizard builds 2 cards going from 7 districts to 9 districts while the Architect builds 3 cards going from those 9 districts to having 12 districts.
It's perfectly OK to build to nine buildings.
The eighth building simply triggers the endgame.
BTW, this is also possible in a multiplayer game using the architect (7 buildings + architect = max. 10 builings). In this way it would be possible to get to 11 districts in a 2 player game.