It is a lot easier to make an abacus from scratch than a slide rule.
(Which is what they did in the story, IIRC. Make a lot of abacusses (abacussi?) that were operated by the whole crew to compute a course correction ASAP to get them out of the tail of a comet after the mainframe went down. Which is a good argument for having redundant systems plus backup systems for anything critical when you can’t just pull over and call AA.)