Specifying tmpdir in innobackupex command line can override the ones in my.cnf, so to specify tmpdir and target-dir for xtrabackup - for its log files - the one specified under [xtrabackup] is used.
Now, as far as TMPDIR above is concerned, yes, it was hardcoded to /tmp; yes, it can be anything since it mainly used for pid files etc. and /tmp is present on all systems, it was hardcoded as such.
Specifying tmpdir in innobackupex command line can override the ones in my.cnf, so to specify tmpdir and target-dir for xtrabackup - for its log files - the one specified under [xtrabackup] is used.
Now, as far as TMPDIR above is concerned, yes, it was hardcoded to /tmp; yes, it can be anything since it mainly used for pid files etc. and /tmp is present on all systems, it was hardcoded as such.