The answer in part lies in the more interesting factoid to come out of the recent migration data. Allegheny County showed net positive migration for the first time in many decades. It was not a big net migration number, but the point is that it was not a big negative number which had been the pattern going back longer than it matters to count. Back in the 1980s the region was seeing tens of thousands more leaving than coming each year, yet there was still new construction. Why? Because there was still a big flow out of the urban core and into suburban municipalities, many of which were building out rapidly despite the overall regional malaise.
So there is a certain Zen to how all the numbers work out (isn't there always?). Now the population decline is at least temporarily abated, but with Allegheny County holding it's own the big source of housing construction in the suburbs is diminished. Plenty of places in the region's traditional centers that could accomodate a lot of new residents without a lot of new construction. Probably a lot of investment needed in existing housing along the way.