I'd like to point out that this article neither claims nor provides evidence that Zillow has lost money.
For one thing, as discussed previously[1], Zillow earns commissions and fees when both buying and selling houses, so they may be making money even when the prices nominally show a loss.
For another, even if the majority of the houses were sold at a loss, that statistic by itself tells you nothing about Zillow's overall profit or loss. For example, they might have sold most of the houses for a very small loss, but earned large enough profits on the others to balance those losses out. There simply isn't enough information to say.