I don't have the article you mentioned but as I understand it the term extreme is reserved for only the absolute lightest hypos... pale silver/grey bands and dark ruby colored eyes. The hypos that are light but not the absolute best of the best are called ultra lights and of course regular hypos are anything form a brownish purple to nearly black.

I think extreme isn't a separate form of hypo but actually a line bred strain of hypo. People selling a "Hypo het extreme" are wrong as extreme and hypo are just different levels of the same trait. (if I remember correctly anyway)

Personally, I think the term extreme is very much overused these days, people no longer save it for the lightest animals. I see critters for sale of FB all the time labeled as extreme when they clearly aren't. Though I suppose such a trait is subject to every individual's interpretation.