The Muratorian Fragment is said to be evidence that the Gospel of Luke was in fact written by him:
Tradition unanimously affirms this author to be Luke. This is attested by the early heretic Marcion (who died c. AD 160; Luke was the only Gospel in his canon), the Muratorian Fragment (a list of the books accepted as belonging to the New Testament; it is usually held to express Roman opinion at the end of the second century) — Luke, Tyndale New Testament Commentaries
Why is this evidence? Why does the fact that it's included in an early canon such as the Muratorian Fragment automatically provide validation for Luke's authorship?