What if they don't allege that the issue was caused by your modification? What if they just point out to the sentence I quoted and say "your modification changed the specified fuel capacity of the vehicle, therefore according to the terms you agreed to, your warranty is void"? It's not like all the conditions they list have to be fulfilled for them to deny warranty, it's just any one of them.
Yeah, you can argue the law with the dealer all you want, but will it get you anywhere?
For example, PayPal has a generic policy where if you start a transaction dispute which results in a return, you pay return shipping. I received a defective product (literally missing parts), which according to the consumer protection law and postal law of the state of the merchant, requires the merchant to pay return shipping. Did PayPal care? Not one bit. They just kept on illegally insisting that I pay return shipping.
The law is only as good as how far you are willing to go to enforce it.