The kernel can and does detect page faults in kernel space. When the kernel tries to dereference a null pointer, the oops you see is due to the page fault. The same thing would work with the invalid page after the end of the stack (that's called a "guard page").
The earlier comment really meant that the kernel is not set up to handle a page fault in a virtual memory fashion -- i.e. do a pagein and continue as if nothing had happened.
But, unfortunately, the guard page has the same problem as 8K stacks -- requires an extra 4K per thread of kernel virtual memory address space and requires 2 contiguous virtual pages. There was a time when virtual address space was in abundant supply and we just worried about real memory, but today the reverse is often true.
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