I think the other distinction there is that a "full stack developer" might have a comfortable understanding of how all these things work in quite a bit of detail.

The understanding does not mean that they want to take the time investment of recreating all that work from scratch.

For instance,

I understand branch prediction, cache lines and inter-processor synchronization. But I'm no kernel developer.

I know what the assembler does, and its goals. But I haven't made one, and any I made would be sub-optimal

.. This proceeds through compilers, operating systems and up to the web stack.

I mean, writing a renderer that fully implements "HTML 5" along with ALL relevant standards - that's a massive software effort. I barely understand it, but I understand enough to get the job done.