It means the libraries are only loaded when they are needed, so if you never use the (e.g.) xz compression feature, the xz library will not be loaded, and a bac

It means the libraries are only loaded when they are needed, so if you never use the (e.g.) xz compression feature, the xz library will not be loaded, and a backdoor added in the xz library simply can't trigger. (Another side note is this may change the initialization order of libraries--so the initialization functions of an xz library don't run until xz is first used, and this may fail to let you intercept the ssh routines in time.)