Mutating with SELECT

While that specific example was an error in the article, yes, it is possible to mutate data with SELECT. SELECT may invoke stored functions - written in C, SQL, PL/PgSQL, PL/Perl, etc - that mutate data. SQL and PL/PgSQL functions invoked from SELECT may use any SQL, including INSERT/UPDATE/DELETE but also DDL. Other languages may use the SPI to affect the database's state and contents.