There are two meanings of “built-in” here, although they both mean “part of the interpreter”. The library reference uses it to indicate that a function is available without an import (it is “not part of a module”, although see builtins). The interpreter itself uses it to indicate that a function is implemented natively rather than in Python (in CPython, it is at least nominally implemented in C).
There is yet another meaning on the C side: an extension module (i.e., one written in C) is built-in if the Python binary incorporates it rather than loading it if and when needed.