At the moment, it’s unknown if there was ever a real-world person behind this username or if Jia Tan is a completely fabricated individual.
Well, I’m pretty sure it wasn’t a platypus. There was definitely one or more real-world people behind this. I doubt anyone thinks that their name is actually Jia Tan, though.
IANAL, but I feel like if the heirs to an estate cared enough about the deceased’s Steam account enough to get the court involved, Steam wouldn’t have a leg to stand on. But that’s probably what it would take to get them to do the right thing.