None of the apostle Paul's books are named in the letter. Hebrews is not mentioned in the letter. Frederick Fyvie Bruce has no textual evidence Hebrews was left out of this letter.
Frederick could have similarly argued, based on from Codex Claromontanus and with better textual evidence, that Philippians, 1 and 2 Thessalonians, and Hebrews were omitted from the Canon. He would have been equally wrong.