[sorbus]

The mass of a single base pair is about 1.08E-21 grams. That's 1.85E10^21 bits[1] of information in a single gram of purified DNA, about a forth of a zettabyte. So, if they're using DNA as the storage mechanism (the slideshow linked in the article indicates that they are), then 90GB is pretty insignificant. Sure, the bacteria will all be pretty filled with DNA, but it's not especially outlandish. Throwing compression at the information (as the slideshow discusses) makes it even less outlandish.

[1] Each base pair encodes two bits, as DNA and RNA is basically a base-four sequence (when we're thinking about it as data storage).

[possibilistic]

If we were to use yeast, we could additionally include methylation. We don't have to stop there: we could encode information in histone acetylation states, transcription levels, etc. to increase this density even further. Granted, how practical would that be?

Using the regulation machinery might be an interesting way to decrypt messages, if it were sufficiently complex a signalling pathway...

[dnautics]

Yeast don't methylate.

Histone acetylation sites? Epigenetic information is too transient and is not necessarily passed down in a 1:1 fashion, which is really bad if you're trying to store data reliably.