A couple of thought: Thousands of similar ELTs are used every day without incident, but that doesn't rule out the possibility that this one made it off the assembly line with a defective battery or diode.
Many high-strength aluminum alloys -- for example 7075 -- are artificially aged at around 300 degrees Fahrenheit. Heating them to or above this temperature will start to affect their mechanical properties, possibly quite significantly. There are stories floating around about aluminum scuba tanks that exploded because heat lamps were used to cure a new coat of paint.
I don't know whether aluminum or composite would resist fire damage better, but it's not as simple as looking at the melting point of aluminum.