The treble roll-off in the Cobalt is a conscious choice made by AudioQuest as it is a function of the filter option they selected to use with the ESS 9038.
The ESS 9038, like most modern DAC chips (including the 9010 and 9016 in the Black and Red models respectively), has several selectable (or configurable) internal filter implementations. Some DACs let you switch between them yourself, some don’t. Absent an app, or a button, there’s no interface to allow this on the Cobalt, so you’re stuck with what the designer selected.
The profile for the Black and Red are more typical, being fast roll-off filters and differ mostly due to them being different ICs with different output stages, but they’re properly extended for Redbook purposes.
Now why did AudioQuest choose the slow roll-off filter?
That’d be speculation on my part.
It could be they wanted it audibly different from the Red and the Black. This certainly achieves that.
Perhaps they have data that shows the DragonFly units are most commonly paired (by a disproportionate factor) with headphones that have treble issues or are otherwise considered bright, and they wanted to take the edge off there.
Maybe it’s an attempt to appease the masses … whereby younger listeners seem to tend towards wanting a less treble-happy presentation and older listeners wouldn’t necessarily be able to hear the difference anyway.
The upper-treble on the Black is a bit uncouth and uneven at times, and can sound strained and grainy and is frequently described as bright (not uncommon when there are treble issues). The Red is smoother and cleaner (which may be down to the different output stage as much, or more, than the different DAC IC used vs. the Black). Though it, too, tends to get described as being a tad bright more than anything else.
So my personal thought is that AudioQuest wanted to get away from that. And in that regard I think it’ll be the general perception that they’ve been successful.