Just to twist the tiger's tail....

[Tobus]

Non tartan kilts have a long history,, so that's not it.
Short yardage has a long history so that's not it.

To me it's overall appearance, not wearing a sporran is a major fault, as is length below the knee or way add those two together and I doubt it's a gentlemans kilt.

On a lady with with no sporran, it's a kilted skirt, what ever the length.

I tend to agree with this line of thinking. Tweed kilts have existed since at least the 1800s, with plenty of examples, not to mention the Hodden grey kilts. It is also said that the original "little kilt" was a low yardage (4 yards, likely) kilt. Our modern 8-yard knife-pleated kilt has never been the only standard to exist.

Even sporrans have not always been standard. WWI military kilts were broadly worn without sporrans, and often covered by a non-pleated over-apron with only a large pocket in the front.

This is an old debate, and not one we're likely to settle here. People will always find exceptions or outliers which defy classification. I'm tempted to simply say that if it's a skirt-like garment worn by a male that extends from the waist area to the knee area and has pleats round the back with flat front aprons in the front, it's some kind of kilt. Beyond that, we're into subclassifications.