Note that the phrase isn't "you can never be too flattering."
This interpretation does make it a very vague instruction ("okay, so what's the 'kind' thing to say when someone shows me a project I think is poorly done?"), but it's pretty damned obvious to anyone but the most stubbornly obnoxious that the kindest thing is never "rip it to shreds with cutting sarcasm and mockery".
It's certainly possible to do even when you have important negative feedback to offer.
No that doesn't excuse being a dick but if I have the choice of getting honest but harsh criticism of something I've built versus somebody grinfucking me, I'll take the harsh criticism every single time.
Also, the article is about giving feedback, not taking it. The fact that you prefer to take harsh/honest criticism rather than overly-friendly/dishonest feedback is irrelevant (and is also a false dichotomy). You appear to state this to refute any a need to be concerned with delivery at all; your personal preference doesn't change the reality for the rest of society.
So, regarding giving feedback, if your objective is to improve the recipient's chances of success, it behooves you to communicate your (honest) feedback in a way that reduces as much distrust as possible. Techniques for this are given in the article.
But again, I agree, honesty underpins all of this.