Possible...
I’m trying to figure out how the wire was still attached to the phone - if the fire was hot enough to melt all of that - how could the flimsy (even the better OEM kind) not be literally charred to nothing?
But I understand what you are saying — the fuse should have blown or the wire disconnected once it had shorted.
Car seats and misc material are supposed to be flame retardant. Of course, any nonflammable material will catch fire if the fire or heat is hot enough.
If the phone weren't involved, I would have expected it to have retained some residual form and yet it was a pile of goop and metal and glass on the floor. I wish that I had taken a pix of it. It was sitting on my boss's desk for a few days after.
Also, I didn't think to look at the seat for a heating element. Looks like an elaborate fan mechanism (from a random search on the Internet). No doubt a lot of power since it has a heating element (I think). Wonder if the blower/heating element is on a fuse?