Knowing something to be true and being able to prove it are two different things.
<Insert an OJ reference here... >
Oracle needs this letter to show mens rea. That Google acted with a 'dirty mind' or rather they knew what they were doing was in fact illegal (civil not criminal).
If you read the linked document, it goes to show that these documents were already discussed in court where Google, after the fact attempted to claim that they were privileged. Oracle's hired guns in plain English walk through the reasons why they are not privilege.
Also in the document there's a comment from the judge that says...
"Judge Alsup warned Google that at trial “you are going to be on the losing end of this
document” with “profound implications for a permanent injunction.” (Id. 41:5-12)"
and then this...
"After arguing the substance of the Lindholm document at two separate hearings; after asserting
on the merits that the document related only to the Java programming language; after
representing to Judge Alsup that Mr. Lindholm was writing in response to a “question from the
CEO,” not at direction of counsel; after being told by the Court that a good trial lawyer could use
the simple combination of the Lindholm document “and the Magna Carta” to win Oracle’s case
and get an injunction (id. 41:1-4); Google reversed course and claimed that the Lindholm
document was privileged."
Hence this is the smoking gun that pretty much should bring Google to the table to settle this case.