Illinois is a common-law jursidiction. Any judge would laugh you out of court if you claimed an off-hand comment on valuation was an appraisal.
A citizen cannot look at the law and figure out if they're committing a crime, which is why I said the law is unconstitutional.
If you drive by a house and say "this house doesn't look like it's worth shit" you're probably not going to wind up in court after someone complains that said house is worth quite a bit more than shit and you're driving the price down.
Laws actually can be vague while remaining Constitutional. Sometimes it's unavoidable. Court cases can be used to refine the edges.
It's usually more difficult to sue, though. The power is usually given to some semi-governmental group. The Courts go out of their way to find professional regulations constitutional, and the regulators try to avoid claims that might get the entire regulation struck down. That's why Illinois never goes after non-commercial speculation about house prices.
But sometimes they overreach. There was an article in the WSJ a few days ago about some state's Barber Commission levying $50 fines on cosmetologists with striped poles out front -- for improperly implying that a licensed barber was on premises.
This woman probably isn't suing to enforce the regulation, as such. She could sue without the professional regulation, but the law sets a professional standard of care for appraisers that Zillow doesn't claim to meet, so she hopes it will help her case.
Zillow is not a citizen.
They're also in the business of providing these numbers, which is obviously far from the same thing as making a comment while driving down a street. I think you're trying to blur a line that's pretty clear to most reasonable people.