It is questionable just who added the subscriptions, though I think it is most commonly supposed they added by Euthalius, Bishop of Sulca in Egypt. It is readily demonstrable in several cases of the subscription that he did not know much about it. He may not have been intentionally lying, but ignorant of the facts for some reason.How ever the guy who said it was someone who would know more about it. What would be the possible motive for lying.
Here is a good example of the confusion from 1 Corinthians 16. The subscription says, “The first epistle to the Corinthians was written from Philippi...” Philippi is in Macedonia/Greece (Acts 16:12). On the other hand, Paul himself writes that he is in Ephesus (v. 8), a city in Asia (Revelation 1:11), and sends greetings from the churches of Asia (v. 16). He speaks of passing through Macedonia on his way from Ephesus to Corinth (v. 5). So the inspired scriptures themselves demonstrate the error of the 1 Corinthians subscription.
I accept the book of Hebrews as the inspired word of God, and a valid part of the New Testament. To me it only makes for an interesting discussion as to which disciple wrote it, and makes no difference in the understanding and interpretation of the book. If it had, God would have made it clear who was his penman.What difference does it make Hebrews is still in the New Testament. And most Christians accept it as the word of God.