Injecting a soul with erased memories into a constructed entity?
This wasn't actually a secret, and the mages of the association never hid it. People on the mainland who were slightly well-informed knew a bit about it. There were even rumors that the Alchemical Behemoth 'Arle Safir' of the Floating City was created using the residual soul of a fallen Divine Being.
Beyond this, Bartlett knew that those War Golems he admired so much were usually imbued with Elemental Spirits born within the various Elemental Planes. This was because spirits were more loyal and purer, and they would completely obey the commands of the creator. Although they only possessed basic intelligence, they could easily satisfy the needs of combat and slaughter.