The person making that observation was curious as to why it happened. To which Matt Coleville simply responded "because the players are there to play D&D." This sparked a huge discussion about danger level, the openness of game worlds, and whether or not it is ok to have a world where players can run into encounters and areas too dangerous for them.
The interesting thing to me though is that this conversation has very little to do with danger level or whether PCs should retreat and return to encounters/areas later when they're stronger. Despite what many people thought. No, this conversation is about two things: Expectations and Trust.
I'm Here to Play
Every game has base line expectations to it. If you invite people to play D&D they're going to expect to be playing adventurers who go into encounters/stories/dungeons where they fight monsters for loot and experience. If you invite people to play L5R they're going to expect to be playing a samurai drama game where there could be action, but where social interactions could prove far more deadly. If you invite people to play Mutants and Masterminds they're going to expect to be playing a team of superheroes, around the power level of your average X-Man in some sort of city.
There is more to the base line expectations of each game, and there are base line expectations people have based on their experience with the GM and other players. But they're what folks expect. It's why we have qualifiers for other things. A "Birthright" style D&D game is very different from a D&D game. A "Crab Wall" game of L5R is very different from a normal L5R game. And a "Teen Hero" while only slightly different on the surface from a normal super hero game also means we're bringing in social and age related aspects that aren't there.
Notice though, with the exception of L5R where the social can be super important, is the base idea of "assess threat before engaging" a key part of the game. The reason for that is because it's not. If you run a power level 8 teen hero game, and put a power level 20 enemy in front of your players with no warning and wipe them with that. That's on you, not them. The same is true with letting level 5 PCs wander into a Level 12 area in D&D.
By default the expectation is that the major challenges/obstacles put in front of the players are challenges/obstacles that they can overcome now. Does that mean you have to do that in your game? Not at all. But you do need to communicate that that is what you're doing.
In general, the best way to set expectations is to communicate them to your players, discuss them, and come to a decision. If you want to run a game where the players will be expected to assess when they may be in over their head, withdraw from those situations, and return when they're ready you need to tell your players that.
You also need to listen to the players as well. It is possible someone isn't looking for the "Shadowrun" experience of constantly risking being in over their head. It is possible that for whatever reason a player wants to be taking level appropriate jobs, going on level appropriate quests, getting into level appropriate fights, and receiving level appropriate rewards for themselves. There's nothing wrong with this. It's how most games are designed to be played by default.
A lot of groups won't care. As long as they know what is going on with the game, then that is fine. But they need to know, or they're going to assume things are one way when they're not and well, that could be the end of the game.
This is only tangentially involved with expectations, but trust is a big part of every table top game. The players need to trust the GM to run a world, adventures, and encounters that are appropriate and fun. The GM needs to trust the players to play into the world/adventures/encounter. Everyone needs to trust the other to not be cheating or breaking the fantasy of the world.
Expectations play a huge part in trust. If you break expectations. If you violate expectations. You lose trust.
If someone trusts you to be doing one kind of game, and you run another, you can damage that trust. Maybe not a lot. Maybe not irreparably, but you still damage it. And if you lose the trust of your table....the game may as well not happen.
The saying goes that "No D&D is better than Bad D&D" and D&D could be any game you like there. It's true too. RPGs are great. I love them. But a bad one is not only a waste of time, it can be emotionally, mentally, and socially damaging. And that's not cool.
So set expectations. Play to those expectations. And if you want to change those expectations, discuss that with your players too. You'll have better games.