I've been gentle parenting and not disciplining and all the bad millennial parent stuff. I never once instructed my kid to say please and thank you and yet since I set an example of doing those things and I conduct myself in a manner that is prosocial my kid also does those things and has never been described as a little monster. They're almost 3 and I've not experienced this "terrible twos" thing.
Not that they'd deserve to be called a monster or anything similar if they were the most misbehaved little kid ever since, you know, they're a kid.
People say you can't reason with a toddler but they (all toddlers not just mine) understand a lot more than people give them credit for. They understand when you're talking about them as if they aren't there and will respond accordingly. They are people with a different set of priorities to yours that's it.