Not to me it isn't.
Why is the Gospel of Matthew any more valid than the Gospel of Phillip?
L
Well, the gospel of Philip was from around 300 AD, and is cleary a “Gnostic Text”, which is why it was not ever considered an orthodox Gospel. The early Church (the orthdodox) clearly viewed the Gnostic movement as heretical and after Marcion demanded that the Church of Rome only accept Luke’s Gospel as canonical, along with 10 epistles of St. Paul, he and his followers were excommunicated at Rome in 144 AD. Within 35 years, St. Ireneaus of Lyon in his work “Against Heresies” would be the first orthodox Church Father to state that the Church only recognized 4 canonical Gospels, MT, MK, LK, and JHN. The other “so called” Gospels that were written in the 2nd and 3rd century (Thomas, Peter, Judas, Philip) were written by Gnostics who were not in communion with orthodox Christians and the Church of Rome, which St. Ireneaus (ca 170-175 AD) stated “all churches should agree with this Church [Rome] because of its premminent authority”
As I stated earlier, the four-fold Gospel was recogized by the end of the 2nd century and the 27 book NT was decreed by the councils of the Church in the late 4th century.
Hope this explanation helps.