My unofficial personal definition is a millionaire is somebody who could liquidate all his assets and have more than a million dollars (US) as a result.
So the guy in the OP wouldn’t count. Not would somebody who had a house with an assessed value over a million dollars unless they could actually sell their house for that amount. Nor would somebody who has a million dollar comic book collection unless somebody would actually pay that much for it.
My issue is whether you have to split up the million dollars. If a couple have a million dollar’s worth of assets, are they both millionaires? Or do they average out as five-hundred-thousandaires?