After reading JohnT's wise words I'd like to add a couple of my own.
I have done quite a bit of on-line searching for any information concerning the science of reverse thrust and haven't found anything. Guppy's on-line references don't discuss it in any depth. It may be that there is something in Kerrebrock's classic text, but I don't have a copy. Maybe someone who does can tell us what it says? I don't have the Rolls book either; something might be in there.
In my experience, persistent quibbles over terminology are symptoms of fundamental difficulties with concepts. Everyone says "oh, it's just words", but in fact it is much more than that. I am involved in international standards work, and a huge amount of drudge work is involved with terminology. It's like bookkeeping: everyone wants to do "real work", but any real work is impossible without it. To my mind, the reason is that terms are used because they denote concepts, and if your concepts aren't clear, then neither is your understanding.
Where we all agree is that the idea, found in many "explanations" of reverse thrust, that braking is mainly achieved primarily through the exhaust, is misleading. I think HN39, I, and others agree that the braking effect is achieved through momentum reduction of some air mass.
Now, here is where terminology becomes important. With which engine component or components is that momentum-reduction primarily associated? The answer so far seems to be: nobody here really knows.
Isn't it worth finding out? I think so.
Maybe DevX, who seems to work for or around RR, or Turbine D, could help?
PBL