But on the other hand, the equivocal Brit'sh reference, the Oxford Dictionary, gives the plural of abacus to be abacuses. As odd as this sounds, it is more in tune to the actual roots of the word, which are not at all Latin, but Arabic. Although the Arabic pluralisation is not used, I believe it's reasonable to say that it doesn't make sense to pluralise a non-Latin word as though it were Latin, masking its etymological roots. For this reason, I choose this plural in this document."

To further cement this interpretation I also quote from the online Oxford Dictionary for 'abacus' at https://www.askoxford.com/concise_oed/abacus?view=uk which states,
"abacus
/abbkss/

• noun (pl. abacuses) a frame with rows of wires or grooves along which beads are slid, used for calculating.

— ORIGIN Greek abax ‘slab, drawing board’."

Hope this helps to resolve this minor slip.

It can't be a slip if it is supported by a widely used dictionary. It is a matter of discussion or, perhaps, of a controversy, but by no means a slip.