You may be motivated to do something when you find the new colleague still in college is earning as much as you do after 10 years just because the company wants to promote women and "reverse the wage gap", so they give female college students positions and wages equal or higher than top rated senior engineers that are male. Just saying, I saw many such cases.
I don't know you, but I have known a number of people who said things that sounded like that. (With different concretes.) They were unable to productively work with people who they thought had been unfairly promoted. Their resentment hindered their productivity. And I found myself in disagreement with their self-image about how good and deserving they were.
It didn't matter whether I privately agreed or disagreed with the promotion that they complained about. The fact that they were looking around the workplace and thinking about that lead to the negative behaviors for themselves. And their negative behaviors added toxicity to the workplace.
For this reason and others, I prefer the advice in the letter. Comparing with others might work out in the short run, but in the long run it doesn't seem to work out in our profession.
j/k of course, I agree, comparing yourself to others is not beneficial. Compare yourself to your past self is more motivational. There's a good chance that today you are better at whatever it is you chose to do than last year.
Furthermore, I'd also advise parents not to compare their children to other children. (it happens)
First, comparing to others will motivate you to get better, have a positive target you want to reach: "I want to be as good as Jake" was my model when I was in college, Jake was at that time the best IT expert I met for a decade.
Second, because you need to know where you are and what is going around you. You need to know your peers and how you are qualified versus them, how productive and appreciated you are and how are you paid. You may be motivated to do something when you find the new colleague still in college is earning as much as you do after 10 years just because the company wants to promote women and "reverse the wage gap", so they give female college students positions and wages equal or higher than top rated senior engineers that are male. Just saying, I saw many such cases.