The problem is that the literature regarding crucifixion prior to the gospel accounts has to do with various forms of suspension but there was no specific terminology regarding the instrument used..
There was a group of punishments that were carried out by a limb suspension, in which sometimes nails were used, and which sometimes resulted in an outdrawn suffering on some kind of suspension tool. That is as far as anyone can go and be true to the text. Anything beyond that is interpretation not translation.
Good that the Watchtower acknowledges this.