Well as far as service requirements are concerned, Brake fluid should be changed every 3 years or 36,000 miles maximum in order to prevent hygroscopic effect where water ingresses and makes the brake fluid ineffective and eventually lowers the boiling point so low that the brake fluid burns off into vapour during heavy use and turns black and thick ... If it's black then it will have contaminants in it like miniscule amounts of dirt and grit due to this coagulation that attracts it which will inevitably cause scoring to the bores and seals to start leaking as a result.
So... The answer to your question is, YES it can cause the failure
, it uses brake fluid to operate the clutch cylinders... No if's, no but's... It would have failed eventually for that reason... The contaminants will always find their way in the entire circuit to the last seals and settle there.
So, if the brake fluid hasn't been changed as part of a regular service schedule, then they have the right to refuse a warranty claim due to lack of upkeep of the vehicle to manufacturer standards.
The question is: Who services your car and why haven't the changed the brake fluid accordingly?
You can then make a counter claim against the service department/ dealer if they neglected to follow manufacturer guidelines on the service schedule and as a result, has caused failure to the system... Proof is in the service stamp and mileage records in the service handbook along with a checklist that a service department should adhere to when offering a "full service" at the correct mileage/ time frame for the brake fluid replacement.
If you have serviced the vehicle yourself and not changed the brake fluid at the correct period... Then you have NO complaints or recourse towards the warranty company, it's your own doing I'm afraid!
Sorry but, being honest and giving you a definitive answer from both sides of the perspective... I have been on both sides of the fence.
Hope this helps