Are you denying that induced demand is a thing for housing? That everyone who wants to move to Seattle will move there regardless of housing prices, and no one will leave because they get squeezed out of the housing market by new arrivals? Or is there a more nuanced argument that I'm missing?
That new condo allows one more family to live in Seattle regardless, whereas if they were competing with existing stock, some family would probably have to leave. We could play a few rounds of musical chairs to prove that fact.