While true on some level, I don't think this is a very useful statement. The importance of mathematical foundations lies in the extent to which they constrain the features of a programming language.

That extent is not the same for all languages. Many programming languages do not appear to be constrained by anything other than some pragmatic hunch of their designers plus the theoretical limits of computability.

SQL is a mess. The author acknowledged that. But the relational model and relational algebra are more serious attempts at creating a small but expressive theory than many of our mainstream programming languages.