Paulos in the quoted snippet is arguing against a straw-man version of the past, set up as a foil to prove some unrelated point, rather than taking it on its own terms and trying to understand it. This is a common conceit of modern writers, but it does a disservice to readers.
Roman numerals by and large were not used for calculating, but for recording calculations. Those calculations were done (as the very word “calculate” suggests) using pebbles or other tokens, on some kind of counting board.
The point of Roman numerals is to be as direct as possible a representation of the state of the counting board. They are a serialization format.
Think of it as the JSON (or s-expressions if you like) of the ancient world. You don’t run your algorithm by writing and rewriting JSON literals over and over with pen and paper. They are just a record; your computation is done using a different automatic tool.
When someone writes down the state of a graph structure as a big nested JSON literal, you don’t laugh at them for having a uselessly unwieldy written expression.