I may be reading too much into it, but I think I understand the lesson of Keemstar's statement.
Let's put it this way: If it gets under your skin when people tell you to give up, rather than inspiring you to try harder and prove them wrong, then I'm not sure if YouTube is something you're passionate about or able to handle. Who cares what people say whether they are big or small? Things like this test your dedication. If you want to be a YouTuber as a hobbyist or professional you are going to come across people with this opinion. How you react is a reflection of you and not necessarily the content of what you're reacting to. If words reduce your confidence in yourself, or your dreams, then you have to work on your confidence. You won't get far in life if words is all it takes to take you down. There's plenty out there and many that are released just to diminish the confidence of competitors. You aren't ready to be a YouTube or online content creator if this alone, or the actual barriers that are in your way, makes you want to give up.
I'm not saying this view I have is what he meant. I don't even know who he is, so I have no indicator of his intentions, but this is what I take away from observing his comments. I see it as a lesson in disguise. If smaller channels take his comments more seriously than they take themselves, then that's a serious problem that will not only stop them from being a YouTuber, but being anything they aspire to be.