That's possible, but then there would be the question of its similarity to the later event.

I'm assuming that the supernova would have been well-known, had it existed. The episode could have addressed some of these plausibility concerns by presenting it as a mere nova in a less conspicuous part of the sky (they could have even used a real one)--but that could have made it too implausible that even Sherlock could figure out the puzzle.