With all its build options and modules, the traditional monolith-microkernel distinction doesn't really apply. Do you mean something else by "extremely monolithic"?
It's not so much the Linux kernel that's monolithic, but the Linux source code. Since drivers gets mainlined, they become part of the Linux stability guarantee. When a significant fraction of the source code is drivers for specialized devices (i.e. AMD GPUs), the development process takes on the characteristics of an extreme monolith that requires coordination of many different teams.
