Mickey didn't get the Keyblade until after Kingdom hearts...and since this talks of Mickey having the keyblade, it has to have taken place after kingdom Hearts
But MyInnerField is right, The Heart is what holds the memories, so the Shell wouldn't recall ever writing them, because the Heart has all the memories of it! The maker of the Reversed Ansem Reports is possibly Ansem's Shell, and maybe he thinks that his name is Ansem, but later find out that he is merely a shell
The first one's couldn't have been made by Diz, because at the time of the first report, Ansem was whole. He was Body, Heart, and Soul. At the time of the 10th report, he cast off his body, which is when Diz was created
Which leads me to beleive that the Heart (The Ansem we've fought in KH, and in COM: R/R) holds the memories of making the reports, and the Shell continued it's existance on another plain, beleiving it was Ansem, all while possibly being manipulated by his dicsiples, but eventually being helped by King Mickey
And do you remember in COM: R/R? Mickey says he remembers meeting Diz before, but he doesn't know when
Mickey is recalling meeting him and convincing him to erase his data, and not persue darkness, to protect him from being tainted like the original Ansem
It makes perfect sense if you think about it. Mickey finds somethign familiar about Diz. Perhaps he recognizes him from the events spoke of in thes new reports?