leave Hollywood a side
It's quite a waste of braincells to speculate what an Alien can or cannot withstand to according to a movie that stays "away" from the origins.
If you can kill an Alien with a flame thrower (shown on second movie and nearly all games) then you can kill it even easier with tons or heated metal. It simply wasn't enough to do so in the third movie, because the director thought it would be "cool".
Points of contention
The initial article refers to the molten metal as being iron. Both dialog and visual evidence contradicts this.
The metal was refered to as "tons of hot lead." Furthermore, iron incandeses while in its liquid state.
Lead will melt at 327 degrees C, about 3 times the boiling temperature of water. A human could withstand those temperatures for perhaps half the indicated time. They wouldn't live very long afterward but that temperature for the indicated time would not be instantly fatal.
The hydrostatic pressures on the other hand speak volumes about the strength of the alien's exoskeleton
flamethrower vs. molten lead
Well one thing to consider is that a flamethrower is designed to burn.The ingredients such as:
napalm, which causes the flame to stick stick the object and burn at a slower rate, or white phosphorus (I believe it's called) which will continue to burn indefinitely. I believe that this means that the temperatures produced by a flamethrower could easily be higher than that of molten lead, but iron on the other hand is a different story.