Earlier quoted context omitted.
Using the same logic, they could subpoena the grave that the body is buried in as the existence of such grave is a forgone conclusion (a shovel and dirt found in the car, a hole in the ground must exist) and thus would not be a testimony. The accused in that case is also a witness in that they witnessed the grave where the body rest. If they lead the police to a different hole then analyze of the dirt can prove conte…
There's no pretense that the prosecutor isn't asking for the content. Of course they are. It's just not a Fifth Amendment violation for the government to look at content you've written, even if that content incriminates you.
In the hard drive case and the hypothetical body location case the government is trying to compel speech that directly leads to incriminating evidence. That seems like a solidly 5th amendment issue to me.