I agree that it’s unlikely to be one of the giant kings based on the physical differences.
I found out as I was researching this, that the location of the Exiled lands is unknown. Some people think it’s near Hyrkania, some think it’s near Stygia, some think it’s not in Hyboria at all. Funcom has been purposely vague on the subject. But, based on the marvel comics, the giant kings were definitely present in Stygia. Conan encounters one near Khemi and surprisingly, it seems to understand Stygian!
Now that is interesting, because it has been established that the Stygians came from a continent to the east of Thuria. Does that mean that the proto Stygians for some reason totally abandoned their old language upon arrival in Stygia, adopting the language of the giant kings, presumably after conquering them? Or did the Stygians encounter the giant kings elsewhere in the distant past? Or did the giant kings learn Stygian as they were being slaughtered? Were the giant kings wide spread and not just in Stygia? This calls for more digging!
Another interesting thing to consider is that nothing actually says where the Lemurian admiral found the artifact. Only that he didn’t want to take it back to Lemuria and built a city to hide it. Does that mean that he built the city right where he found the artifact in Khitai? Or did he find it and then go somewhere to build his city in a place that he thought nobody else would come looking? Interesting questions, eh?
Anyway, back to the topic at hand, lol. We know that the person who discovered the artifact was a Lemurian admiral. We also know that the Lemurians were an advanced race of people. It seems likely that a Lemurian admiral would have had the capability to sail to Stygia. Especially one who was on a quest to find an artifact that bestows immortality. I would go to great lengths to obtain that, wouldn’t you?
But, was he on a quest for it or did he stumble across it? In the Funcom description of the dungeon, it says he found it by luck. That doesn’t necessarily mean that he wasn’t looking for it, but it makes it sounds like he wasn’t. What would a Lemurian admiral have been doing so far inland if he wasn’t specifically searching for the artifact? Admittedly, the cataclysm drastically changed the geographic layout of the world, but the main continent seems to have survived. By which I mean it doesn’t look to me like either Hyrkania or western Khitai were under an ocean during the Thurian age, so what would an admiral have been doing there? I’m guessing he found it somewhere else and took it to Khitai afterward. I’m not a Conan scholar though. I could be way off!
So yeah, the physical differences between the artifact and the giant kings are just too great. So, as you said, 0% chance of being a giant king!