I believe his TEMP tablespace is the true temporary tablespace - created with the "CREATE TEMPORARY TABLESPACE" command. It has tempfiles instead of datafiles and it's possible to offline and drop tempfile without any problems - if there aren't active transactions.

In a "normal" tablespace with datafiles you cannot drop a datafile without dropping the tablespace but this is another issue.

The technique I posted is the least painful I've found for "shrinking" true temporary tablespace.