I Tertius, who wrote this letter, greet you in the Lord.
Romans 16:22 ESV

This verse is not saying that the words of the letter belong to Tertius, but that he served as Paul’s amanuensis or scribe. Tertius was probably a fellow believer, but his identity is otherwise unknown with any certainty.

The Biblical authors frequently employed scribes to record their teachings. Paul, Peter, Moses, Isaiah....probably many more.