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From reading the source of Python/marshal.c it seems that the read function's raise an exception on I/O errors, but don't return a specific value (that is, sentence starting with "It appears that" is wrong). 
PyMarshal_ReadLongFromFile calls r_long, this calls r_string without checking for errors and calculates the return value from the buffer passed to r_string. On I/O errors the buffer may not have been filled at all and contains random data (whatever happened to be on the stack).
Likewise for PyMarhal_ReadShortFromFile (through r_short instead of r_long).
r_string does raise an exception on I/O errors or short reads, but reading from FILE* and Python objects.
The most straightforward documentation update would be:
* Remove the entire XXX paragraph
* Add text to the documentation for PyMarshal_ReadLongFromFile and PyMarshal_ReadShortFromFile: On error sets the appopriate exception (:exc:`EOFError`), but does not return a specific value. Use :func:`PyErr_Occurred` to check for errors.