This summary is not the rule. The rule text controls.
If a document needs a witness signature to be valid, you don't automatically have to bring that witness to court to prove the document is real. You only need to call the subscribing witness to testify if the law that controls whether the document is valid actually requires it.
In most cases, you can prove a document is authentic through other means, like handwriting comparison, testimony from someone familiar with the signature, or other evidence. This rule just makes clear that courts won't demand a subscribing witness unless some other specific law says that witness is required. So the requirement depends on what type of document it is and what state or law governs that document, not on the general rules of evidence.
