"built-in modules" has the specific meaning of modules that are compiled in to the python executable. It doesn't mean modules in the standard library.

See https://docs.python.org/3.6/library/sys.html#sys.builtin_module_names

Python 3.6.4 (default, Jan  7 2018, 15:53:53)
[GCC 6.4.0] on cygwin
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>>> import sys
>>> sys.builtin_module_names
('_ast', '_codecs', '_collections', '_functools', '_imp', '_io', '_locale', '_operator', '_signal', '_sre', '_stat', '_string', '_symtable', '_thread', '_tracemalloc', '_warnings', '_weakref', 'atexit', 'builtins', 'errno', 'faulthandler', 'gc', 'itertools', 'marshal', 'posix', 'pwd', 'sys', 'time', 'xxsubtype', 'zipimport')