It was not made in 2018, that's just the latest renewal of the ruling (which was originally made in 2010). Apple's attempts to make it illegal were denied at the time too, because even without the ruling it fairly obviously falls under exemption.

The ruling also just makes it explicit, the DMCA already doesn't cover things like jailbreaking because they don't relate to copyright, and do relate to the user enabling interoperability of their device with legally obtained software (17 U.S. Code § 1201 (f)). These rulings are not proclamations in contrast with the law, they are bound by it and must be valid interpretations of it. Apple's attempts to classify it otherwise were a clear attempt to abuse copyright law and were rejected as illegitimate.

You clearly do not know what you are talking about.

The Copyright Office didn't construct an exemption out of whole cloth - they cannot do that, and they'd be overruled by Congress if they did. It's a recognition of the underlying fact that the DMCA does not cover TPMs that do not control access to a copyrighted work. The actual text of the law says that.

Not only was it not illegal as the other commenters have mentioned, I believe Jay specifically went to the exemption discussions for it with Cydia as then example of why it should be legal. And this isn't what's being argued here regardless.

Before the decision, jailbreaking was not considered a violation of the DMCA. Apple sued and lost trying to make it one. So your premise that Cydia was somehow in violation of DMCA at the time is completely false.