As originally presented, this dart gun is probably different from the hand crossbow, but that's an unlikely ruling in casual play
Some similar dart guns appear in The Emerald Spire Superdungeon.1 Here, from that adventure, is the original description:
The dart gun functions as a hand crossbow that can hold 20 darts and automatically reloads between shots, allowing the wielder to make full attacks with it. It fires an additional shot each round as if its wielder possessed the Rapid Shot feat. A +2 keen dart gun is worth 9,3000 gp, [sic] and is an exotic weapon that [adversaries that wield them] are proficient with. (145-6)
Because this weapon only functions as (that is, compared to is treated as) a hand crossbow, it has a hand crossbow's statistics except as presented in the text, and the text calls the weapon a different name, so it's not a hand crossbow. There's no indication that the proficiencies are equivalent like, for example, composite longbows and longbows. Additional evidence is in the adversary's stat block: the creatures have Weapon Focus (dart gun), not Weapon Focus (hand crossbow), which, were the latter feat present, would've conclusively expanded the hand crossbow's mandate to the dart gun.
I would totally agree with this answer that the feat Exotic Weapon Proficiency (hand crossbow) would be sufficient to avoid a nonproficiency penalty for the dart gun if that description did say treat as a hand crossbow. Further, were I GMing The Emerald Spire Superdungeon for a PC possessing the feat Exotic Weapon Proficiency (hand crossbow) and that PC acquired one (or more!) of these weapons, that description would be close enough for me to let the PC use it without suffering the nonproficiency penalty. But, if the GM's taking an ultra-strict, rules-as-written, take-no-prisoners, you-get-a-trophy-for-winning position, then, according to the text, the dart gun and hand crossbow are probably different weapons and don't share the hand crossbow proficiency.
Finally, the above dart gun is decidedly different from the typical Pathfinder technological dart gun that requires, instead, the feat Exotic Weapon Proficiency (firearms).
1 I am unaware of other published +1 keen dart guns in Pathfinder products. If +1 keen dart guns appear in other adventures with descriptions matching those of the question, I happily defer to this answer.