The Early Church Leaders took it for granted that Paul wrote the book of Hebrews. That's good enough for me.
https://www.ao.net/~fmoeller/hebrew.html
On another note, I hope they read the very first verse in Hebrews which says:
"God, who at various times and in diverse ways spoke long ago to the fathers through the prophets, 2 has in these last days spoken to us by His Son, whom He has appointed heir of all things"
In other words, God NOW speaks through his Son, not prophets or an organization.