Agreed, the author did hint at this idea with the statement "SQL databases are readily available if you don’t have access to the dedicated messaging infrastruct

Agreed, the author did hint at this idea with the statement "SQL databases are readily available if you don’t have access to the dedicated messaging infrastructure. They also have some unique features that turn out useful while working with message-driven applications." Specialized tools are wonderful but sometimes their use represents a premature optimization when a familiar general purpose tool is good enough to get the job done in the short term.