Note that they also say:

Dolphin is an independently created computer program that is circumventing Wii disc encryption for interoperability with Wii software. According to this exemption, this does not constitute infringement under 17 U.S.C. § 1201. This exemption even allows distribution of information collected through circumvention, like encryption keys, if it is for software interoperability.

17 U.S.C. § 1201(f) is a significant legal protection for emulation in the US, and it is why Nintendo has yet to legally challenge any emulator with the DMCA anti-circumvention clauses despite the law going into effect 25 years ago. Unless a court rules that our understanding of the law is incorrect, we have every reason to believe that our decryption of Wii game discs is covered by this exemption.

Their conclusion is that they believe that they fit in the exemption clauses of the DMCA and therefore, they are not breaking the DMCA by including a mechanism or the key. They think that the exemption clause might not apply if the key or decryption mechanism was standalone.