Earlier quoted context omitted.
It's not bizarre at all. "To be a witness against himself" is not metaphorical or ye olde English. It literally means what it says--the government can't force someone to testify against himself ( i.e. to provide a confession). If the Founders had meant to say that the government can't compel someone to cooperate in an investigation at all , they would have said that.
The Constitution is extremely abbreviated because it was written by hand. Your arbitrary definition of "witness" isn't more or less correct then someome else's; these terms are negotiated over the centuries with a great deal of context. The Fifth Amendment has been consistently interpreted to mean far more than "confession"
?!