Hindsight is 20/20. It’s all about what the right decision was with the information available to you at the time, not what decision would’ve had the best outcome in retrospect.
Making the right decision at the time but later finding out it was the wrong one just seems like a proclamation that it's better to be lucky than smart. I get what you're trying to say, which is finding solace knowing you did what made sense given the scenario at hand, but it ultimately says you can do the right thing and be punished and do the wrong thing and be rewarded.
That said, I still agree with OP - it's good advice not to put too much stock in stock (unless it's liquid).
If you play poker, you know that, for any given hand, you can make the right play but still lose. But you’ll come out net positive by repeatedly making the right play.
You don’t get to play as many hands in your career as you do in a session of poker, so if you’re optimizing for income, generally speaking you should go for a job that gives you the highest guaranteed income rather than work for startups with, say, 1% odds of netting you significantly more than you would get at FAANG, for example.
I agree with you - you're optimizing for the most likely scenario. My point is even if you play your cards right, you still might lose. In other words, it's better to be lucky than good.
That said, I still agree with OP - it's good advice not to put too much stock in stock (unless it's liquid).