WE know the original full-size bell is made from a very expensive niobium alloy, for heat resistance. But I notice that the new bell hardly glows at all - just very faintly at the extreme outer edge. The film cooling provided by the turbopump exhaust protects it completely.
So, I am wondering - are they still using that expensive niobium for the stubby nozzle? From what I can see of the bell, they could use simple mild steel and be perfectly OK.
Full-size engine bells also barely light up at the top, so it might be the same material but shorter.
I don’t suggest that the lack of glow tells us that it is made from different stuff - as you say, that section of the bell doesn’t normally glow - but that the fact it doesn’t glow means that it isn’t that hot, doesn’t need to deal with high temperatures, and therefore doesn’t need to be made from any expensive alloy.