The concept of forgone conclusion is very weird. Imagine that I tell someone that I maintain a diary with log of all the events everyday. Then I tell this to my friends, family, (the police), etc.
Let's say the prosecution can prove that I was at a spot where something illegal happened. If they knew I wrote a diary everyday, they can compel me to produce my diary, which will then be used against me (if needed).
If on the other hand, I tell everyone (and the police) that I have photographic memory and remember anything I see and do, that information is protected by fifth amendment. So in this case, I won't provide something that will be used against me.
It is very weird that when the plain words of the amendment read "compelled in any criminal case to be a witness against himself", it is only thought to include literal production of testimony from mind as confession, while on the other hand, the first amendment is not read literally to allow only freedom of (say) owning a press, the press being a physical printing press.
That's not what the "foregone conclusion" stuff is about, at all. They're not saying "it's obvious he's guilty so the 5th amendment does not apply." The 5th amendment doesn't apply to handing over evidence, period. It's about the circumstances under which handing over evidence (which ordinarily does not implicate the 5th amendment) involves implicit statements or assertions by the defendant (which does). Read the subpoena example in the opinion, it clarifies.