And what does this mean, exactly?
All liquid is going to leave some sort of residue. It's the nature of the flavoring. What's the implication? If there's no/little residue it's safe for an atty? That's a leap to conclusion from the onset. Besides, heating liquid up with an open flame is much hotter than it would get in a PV.
I'm not seeing this as enlightening...
Perhaps you don't understand the physics of vaporization and boiling.
All liquid does not need to leave some form of residue.
"Flavoring" evaporates and/or boils away and is then carried to the tongue, taste buds, etc. Anything that does not vaporize and is left as a residue is "waste". It serves no purpose as far as taste and vapor is concerned, and can degrade the performance of the atomizer. That's absolutely "no leap to conclusion".
As far as this test is concerned, it doesn't matter if you do it with an atomizer coil, an open flame, a soldering iron, or a pan on your stove. Once the liquid reaches its vaporization temperature.. it will vaporize. Period. If anything, this test might be a bit more forgiving than a real atomizer because the elevated temperatures may vaporize elements that might remain unvaporized on a standard atomizer.