No, what the guy told was correct, getting a key and the offering are the requirements to accessing the fight with the overgrowth.
When you get to the boss arena, there is no boss fight, you see Milo (or whatever that farmer is called) and he tells some shit and gives you a key, after that you backtrack a bit and use that key to open the locked door, when you go through that door you parkour a bit and interact with a weird shrine-voodoo-thingy which is "the offering" that the guy was talking about. Once you do all that you return to the boss arena and only then you get to fight "the Overgrowth" boss.