Well I mean it was in the category of "you can catch mew in Pokemon Yellow if you get to the truck behind the ship"-myth. People have said it, I read about it online in the "early days" but I've never seen it happen to anyone.
Yes, "back in the day" you could only get it through official distribution, or hack it in with a gameshark code. There was a truck behind the SS Anne that you could get to(you had to get the HM move from the boss, then faint - that would put you in the nearest pokecentre, so you could continue until you found HM Swim, then you could go back to the ship since it never departed, and swim behind it - and yep, there was a truck) - but that truck did nothing, it was just set decoration.
The RNG method to encounter Mew in the wild was only discovered years and years later and most definitely wasn't known 20 years ago.
