I have only used two different cotis - my third, a vintage, is currently on its way to me.

I believe the Escher which I have, a Barber's Delight, definitely improves the edges of the cotis which I have used. It doesn't improve it tremendously, however it is a worthy improvement. Saying that I need to try doing more laps to define its true limit.

How I interpret the author's quote in the OP is as follows: His new 'Eschers' are not true Eschers, or at least they do not possess the same quality as the Eschers made by Pike and Co that we love so much. They are in fact more comparable to some Thuringians which I am lead to believe some of which only attain edges of the equivalent grit 8-10K. I have read this (can not reference where) and have also read Neil Miller's assessment of certain of his stones for sale at Strop-shop.co.uk. Neil's assessment has been in accordance with some of the literature that I have read as to their finishing abilities.

If what I am guessing is true, then such a stone may not improve the edge over a coticule as the finishes would be very similar in terms of grit size. This is only a guess - I have never tried any Thuringian that was not an Escher, neither have I tried the ones being referenced in the OP.

It is the only sense that I can make of why an 'Escher' would not improve a coti edge, when many including myself have experienced the opposite - unless the seller has only used one coti to determine this, that being the super coti that we are all still searching for....