I'm happy for you. Even a tiny bit jealous. But it does in the end make the whole post sound like a humblebrag to me.
Additionally, you probably can do better investing, but we don't know when he took out his mortgage. I got a mortgage in 2000 and the rates were much higher. A guaranteed 6.5% return on my investment? Not too bad. Refinancing can be expensive, and depending on what remains on the mortgage it may not be worth it.
So really, it's almost impossible to speculate on whether paying of the mortgage was a good idea until we know more of the details. I admit I would be inclined to pay it off just for the certainty and the immediate lowering of expenses. Investing can take time to return, possibly more time than he has.
You're both right of course, and then some of them turn around and say "it isn't about money" or "don't make it about money". Sigh, it isn't unique to tech at all by the way.
I hope I can work hard enough to someday be able to have the luxury of that choice and that I'll remember my current perspective at that time.
And, since this is HN, of course I always need to preface with "Yes, I know you can point to a handful of outlier counter-examples, but they don't disprove the general case".
Single-digit employees more often than not get screwed when considering the greater risk they take (risk not too much lower than the founders, who usually keep several tens of percent ownership). 1% of a company post-seed dilutes a lot after a few VC rounds, so you end up with a great strike price, but not much more %-wise (what really matters) compared to later hires.
Also I think you (as many do, and as I used to) vastly overestimate what retirement money is. In general you can take your yearly expenses (not income), multiply that by around 25, and that's all you really need to retire on, and keep the same lifestyle. (Sure, if you want to increase the cost of your lifestyle, add a couple kids, etc., you need to take that into account.)
I think the reason these posts get upvoted is more because the things that the author talked about are things many of us deal with. I've definitely had times when I stayed awake later than I should have just because I knew that if I went to sleep, the only thing waiting for me was work. I guess sometimes I feel like there just isn't enough time in the day and I don't even get to relax before it's over.
I like to believe that posts like these where the author isn't actually quitting would be upvoted as well. Of course, the issue is that those posts don't exist likely out of fear that the employers would find out.. So then we are left only with the people who have the luxury of leaving writing about these topics.
I deliberately left out actual numbers, but I don't know how to be any more tactful than that. Sorry.