Why do you think it's going to be faster to do a UNION query against 12 (or 13) tables than it will be to do a non-UNION query against a single table?
It's also unclear to me why you're suggesting the author switch to MyISAM. Presumably they're using InnoDB because it's actually ACID compliant, and even if they're not, according to MySQL's own benchmarks InnoDB is as fast or faster than MyISAM for heavy read workloads, e.g. as described at http://www.oracle.com/partners/en/knowledge-zone/mysql-5-5-i...
The UNION query will be faster because you will be doing a UNION against 12 MyISAM tables instead of doing a single query against a InnoDB table. MyISAM is much faster in read only situations. Once you start writing to it it becomes slower, but the 13th table prevents writes, for that 13th table you might need to go with a slow table like InnoDB. ( yeah I know about that benchmark its a flawed benchmark )
It's also unclear to me why you're suggesting the author switch to MyISAM. Presumably they're using InnoDB because it's actually ACID compliant, and even if they're not, according to MySQL's own benchmarks InnoDB is as fast or faster than MyISAM for heavy read workloads, e.g. as described at http://www.oracle.com/partners/en/knowledge-zone/mysql-5-5-i...