If your question is just about getting a shiny
Mew, then it is entirely likely that there are several legitimate ones around; people knew about soft resetting (although that's not to say they haven't been carelessly overwritten, deleted, lost, damaged carts, etc.).
If you mean rng abused
shiny Mew, then I suppose maybe
just about covers it.
As well as what shiny finder said, there's also this thread
which shows information that the exact LCRNG formula had already been found, so it probably wasn't too long after that. There's also the chance that other (likely Japanese) sites had figured it out before this point too, given they had had the game for 8 additional months (imports ignored).
But in any case, it's possible (although likelihood is questionable) that someone picked up Mew event, but never collected their copy (started soft resetting, and lost interest before getting one they wished to keep, for example)... So when they came back to it years later (when RNG abuse was a thing) they still were able to reset it.
Edit: This thread too.