First post here on the forum, but here goes...
I have a few theories about the Heart of Sabik, but they all revolve around spoiler-y content...
Since the finer aspects of forum posting yet elude me, I'll just warn of the spoilers now and pray those who are as yet unaware of the revelations of Shadowbringers will have the good sense not to continue on without due caution.
No, really. Spoilers.
So, for the first, from the way that the shades Emet-Selch created to populate his faux Amaurot behave, a great deal of emphasis was placed upon selflessness, and the fact that an artifact of incredible power, charged with a spell so potent as Ultima, would be named for one individual is worthy of note. While it is possible that Sabik was in fact a city or state within the world of the Ancients, it seems more likely that an individual, either part of the council of 14 or from some other place of prominence, was in fact Sabik. I base this upon the fact that so many places in Amaurot are named rather practically, and unless Sabik was a close knit community centered around a particularly dangerous power source, the term Heart would probably not be appropriate.
This all being said, the fact remains that Sabik's heart is clearly some sort of stone requiring a charge. More specifically, my second, and more likely in my opinion, theory is that Sabik was (again) a famous or powerful Amaurotian or other Ancient, yet in this case I believe that the "Heart" is in fact not a manifestation of the individual, but the culmination of their life's work in the arts of Creation Magic. Specifically, I suspect that the Heart of Sabik is an uncharged form of creation matrix, not entirely different from the one we supposedly receive as a result of the "Community Cohesion" quest. Unlike our (sadly unusable) reward, this one is inscribed with the ultimate combat spell, and needs only the proper aether to be used. If this is the case, then the sad and simple answer to our question is that Sabik once was, and it is now our duty to remember that they once lived.