I think that the loophole is more complicated than the man-in-black just needing someone else to kill Jacob. Because that task in and of itself is VERY complicated. The way I see it, the "loophole" is the method by which the man-in-black can successfully get past Jacob's "protectors" and have him killed, without having to actually kill Jacob himself.
So at the beginning of the episode when they showed Jacob and The Guy In Black sitting alone on the beach talking, did Jacob have "protectors" for him to get past? Seems like there was nobody at all to keep Jacob from being killed. You'd also have to assume that all the years prior to that moment Jacob was just as easily accessible and without "protectors". Yet they still talked about a "loophole" back then. So would a method to get past his protectors really be part of the loophole still?
Also, are we assuming that The Guy In Black was not allowed to kill Jacob? If so, why not?
Yes, I think that the most basic fact in the relationship between Jacob and his Nemesis is that they can't personally kill each other. Either from some "cosmic inability" to do so, or from simple ying/yang due to having equal-but-opposite powers. Whatever it is, it's the rules.
I'm inclined to think that a big part of this hinges on a notion that the relationship between Jacob and his Nemesis is a case of "opposites". From the color of their hair, to where they live, to their powers, to how they think, etc. They need humans as proxies to do things to the other, so, whereas Jacob can use the living, his Nemesis has to use the dead. Further to this, perhaps it would also make sense that Jacob's Nemesis may want to kill Jacob, but Jacob has no desire to kill his Nemesis.
So, the concept of Jacob needing protectors still holds, even though there has yet to be any of the 815ers, or even the crew from the Black Rock on the island yet. Because notice that Jacob's Nemesis mentioned that "it always ends the same way". And then he described how the humans come, they destroy, they corrupt, etc. So, obviously the "game" has been going on forever (or who knows how long). To which, we can assume that Jacob has had many other people on the island before, probably including the Egyptians who built the statue, and who knows how many other cycles of humans who have come to the island, left it, or died there. When the Black Rock appeared on the horizon, the comments by Jacob's Nemesis suggest that a new cycle was beginning.
And so, with each new cycle of humans that comes, Jacob's Nemesis tries to use them to kill Jacob. The missing piece here (among many other missing pieces) is that we don't entirely know what purpose Jacob has for bringing humans to the island in the first place.