You can be sued for anything, of course. A contract has to be enforceable for it to make sense to be sued for it, and California law has a lot of worker protections that make certain common contract provisions unenforceable/illegal. Nuvia's framing is that all of the things he did were previously held in court to be permissible under California law.

IANAL but my reading of the situation would be that apple has a better claim on his IP than the tactic they're taking. All they'd have to do is prove that the idea was formulated at apple and they never signed it over to him. My understanding of Cali law (albeit likely very flawed) is that this suit as written is not supported by law.

Even if the idea that is formulated is for a business that the employer isn't in, and left a decade ago (servers for data centers)? Or if the main idea is "I want to do my next chip design for someone else"? Doubtful.

Doesn't matter IIRC, most employers requires their employees to sign an agreement saying that anything they come up with during the term of their employment is owned by the employer unless the employer gives up rights explicitly. This is to prevent someone from getting "The next big thing" then running away to start a company for it even though their employer was interested.