I wonder if what the words have in common is that their meaning comes from what they're not. You have to have knowledge of a relative antecedent to understand the concepts. They have no meaning without a counterpart or an understood point of reference to compare it to. They're all like word anti-matter.
Foreign -- compared to whom?
Big -- compared to what?
Diminutive -- compared to what?
Incomprehensible -- have to comprehend something first
Untyped -- gets its meaning from not being done
Pulchritude -- have to have a sense of the ugly to understand beauty
S-less -- it's about the absence of s--
Unwritten -- defined by not being something
Indefinable -- defined by not being defined
Misspelled -- have to have proper spelling first
Vulgar -- have to have decorum to have vulgarity
High-class -- has to be in comparison to something
Invisible -- need things to be visible to have this concept
Unvowelled -- need to think of vowels to think of not having vowels
Obscene -- need propriety to have obscenity
Or maybe he didn't have anything in mind and just wanted to pick the most entertaining guess -- if there was more than one winner, would all of them go out? I like the idea of all of them having other words in them, but untyped doesn't seem to be true, and s-less seems to be cheating (also, it has two words in it in that case).