How did they know [SPOILER] didn't survive in Legend of Korra?

I've been rewatching all the old Nickelodeon stuff and something is bothering me. In the legend of Korra, The first season culminates with a standoff between Korra and the season's big bad, Amon. Ultimately, Amon escapes in a motorboat with his brother Tarrlok, who uses an electric shock glove to ignite the fuel tank and blow them both up.

On the one hand, we have seen characters (including Amon) survive much worse. We never actually see Amon's fate to know for certain that he died. That probably has to do more with censorship and the limitations of what Nickelodeon would actually show onscreen. That's not what I'm talking about.

If what we see on screen is exactly as is seems and Amon has died, the only witness is Tarrlok, who is killed with him. To the rest of the world, it would have seemed only that Amon escaped and is hiding in obscurity. However in later seasons, Korra has flashbacks about Amon in which she clearly knows he is dead. Is there any in-universe explanation for how she would possibly know this? Or is it just an error? It seems like a pretty major detail to overlook for a series that doesn't overlook much.