The docs are correct. When you convert a string to a timestamp with time zone it will interpret the string in *your* time zone which is apparently offset from UTC.
Try:
select extract(epoch from timestamp with time zone '1970-01-01 00:00:00-00');
or
select extract(epoch from timestamp with time zone '1970-01-01 00:00:00 UTC');
Note the explicit definition of timezone offset of zero or time zone of UTC.
Cheers,
Steve