"Sometimes it is useful to evaluate an expression or SELECT query but discard the result, for example when calling a function that has side-effects but no useful result value. To do this in PL/pgSQL, use the PERFORM statement:
PERFORM query;
This executes query and discards the result. ..." ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
It discards the results but not the side-effect, which in this case is creating a named portal in the session. While the name of the portal, as returned by the function, is indeed lost the portal still exists, with the same name, and since the name is known by the caller anyway the attempt to make use of the portal succeeds (no failure and the column structure is known). Maybe you are right about the contents of the portal being lost due to the perform but that behavior isn't evident from the the section you quote.