There are a number of questions that are probably answered behind the paywalled article...

* In order to reproduce (and maintain the new bases in its DNA), does the organism need a source of these nucleotides in its growth medium? (i.e. in its food?) The alternative would be to produce them itself, but that would require scarily advanced genetic engineering, I think.

* If one of these bases is in a gene, what happens during mRNA transcription? Total failure?

Point 1 at least should more-or-less ensure this can't escape the lab.