The author seems to miss that relational algebra was developed for the needs of the databases of the time, i.e. in an effort to optimize reads off spinning iron. Any effort for async is destroyed by blocking fs syscalls.
Check out E.F. Codd's paper in CACM from June 1970. https://www.seas.upenn.edu/~zives/03f/cis550/codd.pdf
It starts with "Future users of large data banks must be protected from having to know how the data is organized in the machine (the internal representation)".
It goes on to say, "(The relational model) provides a means of describing data with its natural structure only - that is, without superimposing any additional structure for machine representation purposes".
George Boole (1847, 1854) applied his algebra to propositions, sets, and monadic predicates. The expression p×q, for example, could represent the conjunction of two propositions, the intersection of two sets, or the conjunction of two monadic predicates. With his algebra of dyadic relations, Peirce (1870) made the first major breakthrough in extending symbolic logic to predicates with two arguments (or subjects, as he called them). With that notation, he could represent expressions such as "lovers of women with bright green complexions". That version of the relational algebra was developed further by Ted Codd (1970, 1971), who earned his PhD under Arthur Burks, the editor of volumes 7 and 8 of Peirce’s Collected Papers. At IBM, Codd promoted relational algebra as the foundation for database systems, a version of which was adopted for the query language SQL, which is used in all relational database systems today. Like Peirce’s version, Codd’s relational algebra and the SQL language leave the existential quantifier implicit and require a double negation to express universal quantification.
In fact disk drives were the least common storage system, they were the fastest but most expensive and had the least storage.