Guild of Exiled Engineers – How do you qualify for the Goblin Hewer? (Rules clarification)

I have been reading through the Guild of Exiled Engineers Mordheim warband and noticed something that seems unclear regarding the Goblin Hewer invention.

The Goblin Hewer entry says:

“Troll Slayer engineer only.”

However, as far as I can tell, there are no actual rules explaining:

• how an Expelled Engineer becomes a Troll Slayer,
• whether taking the Slayer Oath has any gameplay effect,
• or whether “Troll Slayer Engineer” is just flavour text.

The only mention I can find is in the Expelled Engineer lore section:

“In some this causes great disgrace, and they take the slayers oath…”

…but there are no mechanics attached to this.

Am I missing something? Was this intended to interact with another supplement or optional Slayer rules somewhere else?

Or is the intended interpretation simply:

Curious how other groups have ruled this, because the idea of a Slayer Engineer dragging around a Goblin Hewer is far too cool not to use