Oracle temporary table usage with XA
For me the wording "Distributed transactions are not supported for temporary tables" is clear: In a distributed transaction you should not use statements that involve temporary tables. This means you should not even insert into an already created temporary table in an XA transaction. Of course it can happen that it works but nevertheless it is not supported so you should not use it.

I found Note 1051543.1 "Known issues with Global Temporary Tables and Distributed or XA transactions"  on Oracle Metalink stating


the usage of temporary tables is not supported in distributed transactions
there are a lot of ORA-00600 errors known when using global temporary tables in XA transactions. Even block corruption ca happen
there are some types of XA transaction where the usage of temporary tables will work. One of such situations is described in the article:


  A global temporary table can be safely used if there is only single branch transaction at the database using it, but if there are loopback database links or XA transactions involving multiple branches, then problems can occur including block corruption (...)
  this restriction on temporary table is documented in the 10g manual . It is not documented in the 11g manual but this is considered as a bug. 
  
  This restriction on temporary table is contained in the 12c manual (you supplied the link).