If it is supposed to be about "sort" then we would have "is" not "are".
First thing, does the sentence sound right to you as a Brit or American? If yes then it is correct. From what generally sounds correct, we can then construct the underlying rules of English.
Example sentences:
People:
The woman who lives there is crazy.
The woman that lives there is crazy.
The woman which lives there is crazy.
The woman what lives there is crazy.
Things:
The book that caused a war.
The book which caused a war.
The book who caused a war.
The book what caused a war.
Analysis: Most people think the third and fourth of both these lists are a bit yucky and so are not correct.
Red33410 is wrong because there is no rule saying that people must take "who" as the subordinator and things must take "that". The rule most people follow is that people can take "who" or "that" but not "what" or "which" (in this usage of "which" - it is of course people to take "which" in "Which doctor do you go to?") and things can take "that" or "which" but not "who".
This of course leaves aside the question of why it would be expected that Dilbert would have "better" (as defined by Red33410) grammar than Scott Adams anyway.
How perfect ... my cheating husband thought he could get away with it ... I told him there are no free rides on carnival skanks, and now he has to buy the ticket. Couldn't believe it when I saw this strip. Thanks for the laugh during a difficult time in my life!