The "you don't" is leaning on the idea that if you actually know what you're doing, an internet comment saying you don't isn't going to dissuade you. If you are

The "you don't" is leaning on the idea that if you actually know what you're doing, an internet comment saying you don't isn't going to dissuade you. If you aren't confident enough to ignore an unequivicol "you don't understand this", then you definitely don't know enough to mess around with this. (The converse, of course, isn't true. There's always going to be someone who is still overconfident enough to ignore it even without having enough knowledge).