At what point do we have to stop considering processed wood as wood, but as a wood/resin or wood/adhesive composite?

Sometime after we stop considering processed food as food, but a food/sugar or food/preservative composite.

Stabilized wood definitely has a lot more in common with resin than with wood, in its mechanical properties.

Your comment reminded me of a technique that guitar makers have been using in recent years, which is to make wood more stable by roasting it in a kiln (which removes the moisture from the wood, and supposedly increases its stability).

If by "recent" you mean developed about 200 years ago, then yes.

From what I've seen, it's become more popular recently (last 10 years).

Its a technique older than guitars, not recent at all. Maybe you just took notice?

well at the very least the technique has been packaged/marketed as “torrefied wood”, and i think that’s what is being referenced here