Yes, it is a contraction for "yet is" (with "yet" concluding the subject clause and "is" being the main verb). It is not a "canned" contraction that you can find in a dictionary. Rather, the vowel sound is dropped from "is" and the "s" sound is tacked onto the end of the preceding word for words where this is comfortable to pronounce. This kind of contraction is common in spoken English, but unusual in writing unless, as here, the writing represents spoken dialogue.
Other simple examples:
- "The ball's red." (The ball is red.)
- "Who's going to the store?" (Who is...?)
- "Bob's at home today." (Bob is...)
This can also be done with other simple verbs like "will" (drop the "wi-" sound and tack the "L" sound to the end of the preceding word) and so on.