Or body seam filler, of a tin solder alloy. I have some for doing body panel work, it's a non-lead substitute that melts at half that of aluminum, maybe around 600 F. Hit it with a butane or propane torch and it easily flows into puddles like that in the picture. But those pictures show evidence that those cars experienced extremely hot temperatures that could easily melt aluminum as you say. Interesting to see the tire steel strands left over after the rubber tires vaporized.
Looks like melted solder I scrape up from beneath my workstation but I think it’s the alloy car wheels.