Extra satellites only have the potential to help a little bit in urban environments by increasing the odds of having a satellite directly overhead. Dense urban environments will have high multipath(longer time for signal to get to the receiver due to bouncing off buildings) which causes the position to be inaccurate.
They hand waved over a lot of it. The improvements in the urban canyon are not coming from GPS, but from things like wifi and 3d mapping, probably including cellular triangulations.