Is this the warranty you have?
https://owners.nissanusa.com/content...s/Extended.pdf
https://owners.nissanusa.com/content...ge%20Guide.pdf
Search for item 30620 in the 2nd pdf. That's the clutch slave. Never heard of a preferred Nissan warranty. Is this back by nissan or a third party dealer warranty?
Rather than taking their word for it, look over the terms of your warranty. If it's not in the exclusions, it should be covered.
Edit: I see you're in chicago, which dealer did you buy from?
edit2: Found this,
http://www.nissanusa.com/content/dam...Powertrain.pdf
I think this is what you have, right?
Your dealer really needs to bat for you. The slave, while not an internal part of the manual trans, is none the less part of the transmission on a manual car. The MT car does not have a torque converter, vacuum modulator, etc.
Contact nissan NA and bitch about this. If it's not covered it should be clearly spelled out as a non covered item like the clutch friction disc is (item 8.2 in the pdf I linked above).