I took that phrase differently. The story makes the point that the AIs fail when metrics of quality can't be expressed in words. The use of a bare "adequate" reinforces the opacity of the coffee's quality. Certainly it would have worked well to use more words to convey specifics of the "adequacy" as you mention, but IMO that would have undercut the link back to the theme of human ineffability.
Obviously everyone's mileage may vary, but I didn't see this as a huge defect, and actually felt it worked pretty well.
Adequate coffee almost works as an image.
In the hands of Douglas Adams or Kurt Vonnegut it could be spun into a whole recurring motif.
In this case it's merely...adequate. Almost captures the density of ideas packed into something like "The ships hung in the sky in much the same way that bricks don't" but doesn't quite manage the same effect.