I had always presumed the advice about use of metals in rockets was more about limiting shrapnel in case of a severe motor cato. As such, Aluminum and other ductile metals may be OK, as they will deform rather than break apart, but something harder and more brittle (like high carbon steel) would tend to break into pieces (like a grenade) which would pose a higher risk of injury (they fly farther and faster than a big chunk of deformed aluminum).