Table Dee

This is the relation that has no attributes and a single tuple. It plays the role of True. (inversely: TableDum)

I wonder... is TableDee the relational equivalent of the SingletonPattern?

Essentially no, because the relation has no attributes. I.e., TableDee is equivalent to a singleton with no members; hence meaningless in the contexts where a singleton is usually applied. However, any RelVar with attributes, defined with a candidate key of KEY {} (using TutorialDee syntax, a candidate key consisting of no attributes) would be (roughly) equivalent to the SingletonPattern, because it enforces the constraint that the RelVar have a cardinality of zero or one.