This summary is not the rule. The rule text controls.
If a document has a witness who signed it to confirm it's real, you don't automatically need to bring that witness to court to prove the document is valid. You only need that witness's testimony if the specific law governing the document requires it.
In most everyday situations, you can prove a document is authentic through other means, like handwriting comparison, testimony from someone familiar with the signature, or other evidence. The witness who originally signed the document as a witness isn't automatically required to show up and testify.
This rule matters because some documents, depending on what type they are and what state or jurisdiction's law applies, might have special rules requiring a witness's testimony before the document can be used in court. If that law applies to your document, you'll need to follow it and get that witness to testify. But if no such special rule applies, you can use other, easier ways to prove the document is genuine.