You're right, I was mixing up notes and memos. Specifically, when I meant to write "memos" I wrote "notes". Thanks. I've corrected above.
But the rest of my post still stands -- "invoice type IV cannot include credit memos and invoice type CN can only include credit memos". ie, credit memos should only appear on a credit note (invoice type CN).
I believe that the above agrees with Gyorgyi's presenation, but that "(credit) invoice" refers to "credit note", which is just a type of an invoice. Thus, I agree with most of your interpretation, "Credit Memos have to be included in separate invoice", but that invoice has type CN.
And I don't believe that this disagrees with A9 (though it could have been made more explicit). "The use of credit notes [ie, Invoice Type = CN] is, in general, confined to: [things that credit memos do]". So, basically, "use invoice type CN for credit memos".
See, for example, the example "Invoice 2" on page 269 (v3, untracked). Element 27 is CN.