My understanding is that it’s just the large nozzle extension that’s made of niobium as it’s radiatively cooled - the upper part of the bell and the combustion chamber is cooled by pumping propellant between the walls, so it can just be made of steel.

This is what the Merlin Vacuum engine looks like without the nozzle extension (though this is 1C): Image

It’s possible that the ‘stubby’ engine is just this.

Full-size engine bells also barely light up at the top, so it might be the same material but shorter.