The guides(the physical ones, the books and shit) come out at the same time as the game. Honestly I don't know how someone can find this without reading the sign. There's no torch around and it's not dark so you're not playing with a torch on, it's literally the only place in the game there's a non torch thing you can light up iirc, it's not apparent it can burn(not covered in oil or tar or something that looks flammable), you can only burn it in one specific spot, there's no logical connection between the mill burning and then stopping leading to draining the poison. It's very possible to randomly do it, but the odds would be very low with the way torches worked in DS2 as far as I remember(had to light them at a torch/campfire and run around then if you changed weapon you had to run back again right?). Considering the signs were up on like the first day, it seems way more likely someone simply read the official strategy guide, then wrote the sign.
Boss is doable either way so it's not entirely gamebreaking, it's just terrible design and with the poison it's probably the hardest boss in the game unless you have a health heavy build and/or are overleveled(even then it's still quite challenging). That was just my original point, the boss jumps out as being hard for wrong reasons, but other than that both games had mostly easy bosses, with some really dumb easy in both but especially in Bloodborne I found. A lot of bosses I beat by literally mashing attacks due to the regen on hit and the auto visceral shit at certain % were pretty weird design too imo where you'd sometimes just go from visceral to visceral and the boss wouldn't move the entire time(a few of them work like that if you do enough damage after the visceral before they recover).