That wasn’t claimed. What was said is the archetype is simultaneity. That is 100% accurate for how the term “market maker” has been used, globally, since at least 1999. (Pre-GLB/LTCM and post-ECN, the term was used more broadly.)

Drift from simultaneity incurs cost and risk. Those costs and risks must be managed. If you aren’t thinking in those terms, you aren’t market making.

Zillow’s downfall mirrors that of the money-centre banks in securities dealing post-GLB leading up to the crisis. What does and does not constitute market making, which is risky but less so than leveraged day trading, was a huge area of policy concern. When non MMs think of themselves as market makers, there is a predictable set of risks they get downed by. Zillow, like so many others, fell prey to that misconception. (There is loose analogy in the ABS markets, where banks holding inventory of esoteric products, either badly hedged or hedged with a busted counterparty, got hosed.)