That's a bunch of BS. I've recommended people don't sell a good stock because they think it might go lower but I've never recommended against selling off a portion when one's position has grown so much it is too large a percentage, depending upon their phase of life and goals, and the value represented by the shares. Never sell a good stock simply because it went up because you don't know how far it might continue to go up.
I've been continually retired for over 20 years and have no other income other than selling shares except for a small amount of dividend bearing stock. I don't sell covered calls for income because I believe that's the best way to miss out on big runs and I prefer to chose the time I sell myself, not leave it to the vagaries of the market. But I HAVE to sell stock occasionally to live, eat and drink, buy properties, pay property taxes, travel, buy cars, etc. I don't sell a little bit every month or even every year because I don't want to be forced into selling at the wrong time. I like to maintain a minimum of 2 years living expenses in cash at all times. Additionally, I've never sold a single share of TSLA without reporting it here within hours of selling. I've never even used the term "diamond hands" as I think it's a silly term.
In other words, I've been practicing exactly what I preach and it has been working exceptionally well. You are way off base here. I couldn't live without occasionally selling stock or going into margin, something I consistently recommend avoiding.