"Subjects" and "pronouns" are not mutually exclusive categories.
The subject designates the actor in a sentence, and can either be a noun or a pronoun.
A pronoun is a part of speech, and does not a priori determine what part of a sentence it occupies (it could be a subject or an object).
So the answer is that, in "who is that," the word "who" is *both* the subject of the sentence and an interrogatory pronoun.