What exactly is textural density??

I happen to have a PHD in Textual Density. My dissertation was on the Rosetta Stone. 

I happen to have a PHD in Textual Density. My dissertation was on the Rosetta Stone.

I just begin to read your dissertation.... Congratulations!

https://www.amazon.com/Timbre-Acoustics-Perception-Cognition-Springer-ebook/dp/B07RMT74YN

Another great catch by Mr Magister!  That link is a great portal into the science behind all this audio psychology.
Music is the MOST abstract of all art forms. The result, when trying to describe abstract ideas, is usually metaphor.  Try writing down how it felt to get your last orgasm AND WHY and you’ll approximate the dilemma faced by any published critic.

Textural density to me means palpable,  having body, 3 dimensionality. I get this “feel” in the lower frequencies as heft, weighty slam .   In the midrange as layering , having mass.  Not so obvious in upper frequencies but discernible as having mass even if the sounds float between speakers.  One way of experiencing this is to compare different systems.  Once you pick it you will understand it’s meaning 

....one thing that struck me over the " a race car with fresh, sticky tires " comment is the fact that cold 'fresh, sticky tires' on a race car can fail to stick in curve six at speed just as fast as the hot worn ones you're just about to pit with....

'Textual Density' in music, Yes, can be thought of as describing a well recorded passage and/or series of same from an outstanding musician/singer or group of same, be it a solo or group performance.  It can also be applied to an instrument in the hands of Yo-Yo Ma, always a delight to hear....

In the case of the reviewer....likely more to the tactical qualities of his brain matter and his noted penchant for 'prose' postures.  We can perhaps assume that he may be paid by the word count and how glossy something is by such flourish pronouncements....

Just an cynical aside, mind you....carry on....;)