Thanks for this. At first glance, it sounds (excuse the pun) to me like this may refer to the viola being played throughout the overture, or leaving it out (or leaving it in) for the entire overture.

Only issue I see with this is the idea of the overture usually being in the beginning of a piece, and the source text here including words like culmination.

But it could also be an overture to a movement (not the entire piece) so maybe the last piece which would make sense given that it somehow seems to represent the composition (composizione) as a whole.

Can you provide the reference of the actual piece of music that this is referring to? I think the best or most likely interpretation to go with is to say that Debussy is leaving in the viola part completely in this overture, whereas in other overtures or passages in the same piece he took it out. But this time since it is a climax or culmination, he uses the "du talon" marking six times for extra volume and emphasis to create drama.

'Debussy leaves theme a hanging on completely alone in the viola part'
There is a point in the last movement where the original Gb-C motif is played six times in the viola, whilst the other parts are silent. I am guessing this is what the text is referring to. It could be just that the motif is prominent, as has been suggested, but I wonder whether it also refers to the idea of the theme left over from the beginning surviving with minimal support.

--------------------------------------------------
Note added at 6 giorni (2021-05-03 09:21:25 GMT)
--------------------------------------------------

An inciso is a motif (or motive), or possibly a figure; both mean a very small thematic element. A motif is (roughly) an important figure, and it looks as if this Gb-C figure is used as an important motif in this piece.