That is actually a really bad rule, though you probably are only joking.
There are many examples of short, but very valuable code. Just think about anything math or physics related.
A totally new or even just a very efficient implementation of an already existing algorithm can be gigantic if others need to build upon it.
And many licenses are verbose not because they are complicated in intent, but merely because they need extensive legalese prose to cover against many possible avenues of attack.
If you have to say it often, it might indicate you have trouble formulating your initial advice in a way that is acceptable to people.
Nobody likes to be told they’re wrong, so it helps to be empathetic about it. Packing your advice or instructions into a tactful and diplomatic approach doesn’t cost you much, but makes it much more likely for your advice to be accepted and implemented. And the recipient will usually end up being grateful for having avoided a mistake. They might even start to look for your and ask advice in the future. And if you keep doing that, he might even consider you a nice person or even a friend.
An arrogant and condescending approach will only do harm, even if you are factually right.