How can Nuvia IP be invalid? Well, that's the beauty of Intellectual Property: it's not really a property it's a weird legal construct. Arm argues the entire Nuvia IP should have disappeared because the license allowing it to exist was voided when Qualcomm bought Nuvia. And Qualcomm had no legal right to move the IP on the foundation of their ARM license. They argue Qualcomm should've negotiated for a new license to use Nuvia IP. Whether they are right or not is up for the courts to decide but they make a legally sound argument without a doubt.

If this makes no sense then here's a simple example: a DVD is not property, it's intellectual property. If it were property then region locks couldn't exist. Compare it to a book. You buy a book in London, no one stops you from reading it in New York, it's your property. Absurdity is the name of the game: in the DeCSS trial it was argued Johansen trespassed on his own computer.

Edit: ah, I have a better, simpler explanation: Arm and Nuvia made a contract. Qualcomm bought Nuvia. Arm argues Qualcomm violated the terms of the contract. That's it.