The paper specifically states that these unnatural base pairs were chosen because they can be handled by natural polymerases. Also, the only way to maintain a plasmid over time is to replicate it. In fact, the whole logic of the strategy is to have Cas9 target mutations to the plasmid so that loss of the XY base pair during replication would lead to loss of the plasmid, and thus loss of the antibiotic resistance gene that it also carried.