You could machine the first 5.9" with scarcely any taper at all - maybe .005 per inch. That means you'd have .0295 worth of taper by the time you reached 5.9" - then you could machine the last .1" with a radical taper of 1" per inch - a 45 degree angle. That would be a lousy forcing cone, but a forcing cone nonetheless. There are a million different ways you could machine a forcing cone. Just look at how many ways chokes have been machined at the other end of the barrel. The total amount of taper may be the same but whether you get there gradually or abruptly is up to you.
The guy may be a crummy writer but I think you're trying too hard to find fault with his story.
