Parent is talking about temporary tables. Those are normally only live for the duration of a transaction (well, session, but in practice if you're using temporary tables across multiple transactions you have a logical application-level transaction which needs to be able to handle failure part-way through). After your transaction the writes to non-temporary tables should be persistent.

Postgres temp tables on ramdisk are a problem for a different reason, the WAL, as pointed to by a sibling comment.