As Dave Black notes, his argument is really not one of internal but of external evidence. Dr. Kok cites Dave’s book The Authorship of Hebrews: The Case for Paul, which I publish. I do note the 2017 publication date, which suggests use of the new hardcover edition, which I link below. The more usual paperback is just $5.99 while the Kindle edition is just $2.99 for those who would like to check Dave’s arguments.
My own view on this is “author unknown.” After reading Dave’s work, I no longer say “anybody but Paul,” but I still find the differences in approach and style from Paul difficult to accept, and I find the absence of Paul’s salutation at the beginning odd, at a minimum. But I put greater emphasis on internal evidence, so this may be simply a difference in approach.