I'm wondering if 'total collapse of cell walls and the complete densification of the natural wood with highly aligned cellulose nanofibres.' means that stuff isn't going to burn until you get temperatures where steel melts anyway?

It would seem so. from the article:

> Massive wood walls and structural beams and columns comprised of engineered panels have demonstrated fire performance equal to concrete and, in some cases, superior to steel

one big difference seems to be that once the critical point is reached, even if the threshold very high, the wood contributes to the fire and nourishes it, unlike concrete or steal. a skyscraper with proper sprinkling should be fine.

Tons of major cities have been wiped out from fire in the early 1900's. But you have to consider how ill-equipped they were to handle fire, not to mention a total lack of research. Modern cities have working fire hydrants and buildings can have multiple fire suppression systems. Most of this is due to those massive fires happening.

Here is NIST's report on fire resistance of timber structures (https://www.nist.gov/sites/default/files/documents/el/fire_r...). It includes guidelines that help increase the fire safety of wood buildings. This page (https://www.firechief.com/2017/03/14/are-wood-frame-high-ris...) shows CLT has had testing to approve it for use in 6+ storey buildings in multiple countries.