Earlier quoted context omitted.
There's nothing saying that increased supply always leads to higher supply in relation to demand, which is what affects price.
So you're arguing that demand increased more than supply in the same period? By your logic, that's the only way that prices could fall while the new housing was built.
In the most basic sense of economics for prices to fall supply has to increase more than demand. If demand increases more than supply the opposite is true. There's nothing saying that a more supply can't also increase demand or even increase demand more than supply.
When more wealthy (or ambitious) people move into an area it often leads to that area becoming more attractive (schools, restaurants, entertainment get better) increasing demand.
Therefor there's nothing particularly strange about arguing that luxury condos could lead to higher rents as the op makes it out to be.