I was confused by the author's definition of a relation. They said that a single tuple is an "unary relation". They probably meant a single-tuple (1-tuple).
Removing the hyphen changes the meaning. https://en.wikipedia.org/wiki/Finitary_relation
The author seems to have misunderstood Codd’s definition (which is just the usual definition for mathematical relations). An N-ary relation is an N-column table (in the idealized database model where a table is a mathematical set of rows, i.e. no duplicate rows).
