"Biodiversity" requires variations in local environments.
Logging, even selective clear cuts, provides those variations as meadows grow where trees once stood (as the replacement trees grow up), with transitional environments to deep timber. That means different plants, and different critters in those localized environments. Homogeneity does not lead to diversity.
The premise is false, the absence of roads just means that when there is a fire, it can only be fought from the air, or by people on the ground with mostly hand tools. Or we all get another Summer of Smoke, like this last one has been.
One more thing: nature is a dynamic system. You cannot 'preserve' a dynamic system in stasis. It's as useful as a rail system that has trains that don't move.