That would be an interesting case but that's not what seems to be argued here. The state is arguing that they have enough evidence that "the presence of child porn on his drives was a 'foregone conclusion.'". It's likely the defendant didn't use forgetting-the-password as a defense because it was obvious via IP traffic and witness testimony that he had regularly and recently used his computer.

The ruling here seems focused on the point of whether the knowledge/use of a password constitutes self-incrimination, which people have a Constitutional right not to engage in. IANAL, but it seems akin to arguing that you have a Fifth Amendment right not to give up a DNA sample.

edit: Looks like I'm wrong, defendant did use forgetfulness as a defense at one point, though that was ultimately not his only reasoning for appeal.

Look at page 7 of the document here:

https://arstechnica.com/wp-content/uploads/2017/03/rawlsopin...

        Approximately one week after the Quashal Denial,
        Doe and his counsel appeared at the Delaware County Police
        Department for the forensic examination of his devices. Doe
        produced the Apple iPhone 6 Plus, including the files on the
        secret application, in a fully unencrypted state by entering
        three separate passwords on the device. The phone contained
        adult pornography, a video of Doe’s four-year-old niece in
        which she was wearing only her underwear, and
        approximately twenty photographs which focused on the
        genitals of Doe’s six-year-old niece. 


        Doe, however, stated
        that he could not remember the passwords necessary to
        decrypt the hard drives and entered several incorrect
        passwords during the forensic examination. The Government
        remains unable to view the decrypted content of the hard
        drives without his assistance.

However, in the next paragraph, the document refers to a ruling in which the court found that there was enough evidence to show that the suspect "remembered the passwords needed to decrypt the hard drives but chose not to reveal them because of the devices' contents". I imagine the details of that evidence was in the Oct 5., 2015 hearing in which the suspect "neither testified nor called witnesses. He offered no physical or documentary evidence into the record and provided no explanation for his failure to comply with the Decryption order".