"If you’re waiting for a building boom to make Portland affordable for you, you’re already too late."

I suppose you are talking about the royal "you," since I already own two houses in Portland.  And of course it's not the Portland I grew up in.  Neither is New York compared to how it was in the 1980s, or the San Francisco, or the Los Angeles.  Cities are always changing.  For better or for worse depends on your relative nostalgia and your relative wealth/income.

Building new units, even luxury, does exert downward pressure on prices across the board, however.

As for how many condos they would have to build to make Portland affordable, they throw out an educated guess based on rental and housing data in this article about San Francisco that you could theoretically transcribe to the Portland market in terms of numbers: https://medium.com/@andersem/a-guy-just-transcribed-30-years-of-for-rent-ads-heres-what-it-taught-us-about-sf-housing-prices-bd61fd0e4ef9#.39ygpl638