Shrinking the kernel with link-time optimization

In this passage :

"When CONFIG_HIGHMEM is not defined, is_highmem_idx() (and PageHighMem() derived from it) return zero unconditionally. Any code within functions that follows the "if (PageHighMem(page))" pattern will be automatically optimized away as dead code.

But this works only because is_highmem_idx() is marked static; ".

Why this works only for static functions ? Static keyword implies only about the internal linkage right?

Thanks