The poll is rigged. It should be Assets - Liabilities. If they don't count Mortgages, they probably don't count Home Equity Loans, Student Loans, and Liabilities, such as Alimony, back Child support etc.
No the Definition is 'Assets - Liabilities' otherwise the statistic is Bogus!
On the other hand, most people deep in dept do not have a lot of liqued assets around.
How are the numbers collected? Lots of investors invest in real estate. Some advertizers contend that most millionaires made their money in real estate.
So someone with 10 million in apartement complexes is not considered a millionaire, but another with a 401K of 1 million and a $900,000 mortgage is a millionaire.
The criteria is a slight variation (probably due to misunderstanding on the part of the reporter) on the definition of an "Accredited Investor" .
Accredited investors are considered by the SEC to be more sophisticated than average, and it is legal to sell them investment products which the SEC does not believe are suitable for most people.
My personal policy is that if I do not understand an investment product, I am not going to put any money into it.
This is not an actual poll, but uses publicly available statistics, mostly the Fed Survey of Consumer Finances. They collect data on every category of income and assets, all the things you mention and many more.
This data can be freely downloaded in SAS or Excel formats by anyone, and you can use it to generate any sorts of reports you like, including or excluding anything. This is evidently what Phoenix does.
The Fed data breaks down real estate assets and mortgages into personal residence and non-personal residence. They also give the mean and median values of credit card balances, vehicle loans, education loans, etc, etc.