> Это как, по всей операционке LZ4 только хаммером2 и используется?

Да, l4z в ядре NetBSD нету.

А касательно реюзинга кода, вот цитата с их сайта:
Probably the primary goal of the NetBSD project is emphasizing correct design and well written code. One example is the implementation of a machine independent bus infrastructure, which enables a single driver for a device (such as an Ethernet or SCSI interface) to be shared across different busses (such as PCI, EISA, Turbochannel), and across different platforms (alpha, pmax, i386, etc), rather than the traditional approach of writing and maintaining many different versions of the driver, each with their own tweaks. In NetBSD, the `tweaks' are in small 'glue' functions that allow improvements to the core driver to benefit all ports.

This also means that, in many cases, a new port is as simple as implementing the machine specific code to access the machine independent bus infrastructure to utilize drivers that have already been written.