Are you asking about the "ramp soldering jig" which goes for just under $100, or their sophisticated "Forster Universal Jig", which goes for close to $500?
The former less-expensive one probably has an aluminum base, but the other parts look to be made of round steel rod. The more-expensive unit looks like an aluminum base also, but a couple of other centering parts which could also be aluminum.
If one was to use a brazing torch to do the job, then steel would be preferred wherever a flame might touch.