Zillow just uses the data it gets from its data sources. I get your point that it should be easy, but is there a standard definition across all 50 states now? They have 30 days to come up with a definition. The EO itself seems to acknowledge the possibility of er, "circumvention", lol. They also have to define "large institutional investor".
By the authority vested in me as President by the Constitution and the laws of the United States of America, it is hereby ordered: Section 1.
www.whitehouse.gov