Investment in commercial real estate is not a recent phenomenon. 

Even "mom and pop" investors have to raise rents; I think this article is referring to people who bought multi family properties decades ago for much lower prices. I'd wager a bet that when they originally purchased the building their rent was "high" for that time period. Point being, prices are relative to rents-no one can buy an apartment building at this time, then afford to rent it out for below market rents. That's because apartment buildings are priced based upon net operating income/ a given cap (return) rate (income approach), so if rents go up, so does the price tag.