Maybeh itz dat the words on da list all mean their opposite:

Foreign: Is an English word and not a foreign word
 Big: IS a small word having only three letters
 Diminutive: Is a big word having as many letters as it has (I’m not counting)
 Incomprehensible: Is comprehensible
 Untyped: Is typed
 Pulchritude: Is not a beautiful looking word
 S-less: Has two s
 Unwritten: Is written
 Indefinable: If definable
 Misspelled: Spelled correctly
 Vulgar: Is not a vulgar word
 High-class: is not a high class word 
 Invisible: Is visible
 Unvowelled: Has vowels
 Obscene: is not obscene