I found the cause of the problem I was having: the add-on product's testing code was foolishly calling C's exit(0) when it though it had finished. However, this is a very slow way to build a list of exit values: a published list would be very useful.
Quoting man xcodebuild:
xcodebuild exits with codes defined by sysexits(3)*. It will exit with EX_OK on success. On failure, it will commonly exit with EX_USAGE if any options appear malformed, EX_NOINPUT if any input files cannot be found, EX_IOERR if any files cannot be read or written, and EX_SOFTWARE if the commands given to xcodebuild fail. It may exit with other codes in less common scenarios.
Otherwise, looks like the previously published list went missing from the mothership years ago, so...
About 65, see:
0 /* successful termination */
64 /* base value for error messages */
64 /* command line usage error */
65 /* data format error */
66 /* cannot open input */
67 /* addressee unknown */
68 /* host name unknown */
69 /* service unavailable */
70 /* internal software error */
71 /* system error (e.g., can't fork) */
72 /* critical OS file missing */
73 /* can't create (user) output file */
74 /* input/output error */
75 /* temp failure; user is invited to retry */
76 /* remote error in protocol */
77 /* permission denied */
78 /* configuration error */
/* maximum listed value */