I am terribly sorry if anyone has already posted/come up with this theory.
I propose that all fabrials used for soulcasting are at best merely sources of stormlight required for "bribing" objects into changing.
My reasons for supposing this are:
1. Fabrials are technology. Therefore everyone should be able to use them like any other fabrial. Indeed Soulcasting Fabrials are an anomaly in this case.
2. The scarcity of people who can Soulcast is strange-they must be hired from the king at high price whenever a highprince requires them. Some countries do not have any-such as Kharbranth.
3. The secrecy surrounding their use is suspicious,especially when Kabsal mentions in chapter 42 that "You just put one on, press your hand against something, and tap a gemstone with your finger. It works that simply."
. Both Shallan and Navani assume that the Soulcasting fabrials are from ancient times yet Dallinar mentions in chapter 61 "The ancients didn’t have fabrials, Navani. I’m certain of it.”
Therefore, I conclude that those people who claim to use these fabrials to Soulcast actually share Jasnah and Shallan's gift.