The key to this is 1975's comment about them coming late in his career. At that point, you don't take risks on the money unless you have a big chunk of change already safely tucked away. And if you're still paying a mortgage at that time, it's unlikely you have a large enough safely net. Last thing you need is a 30-40% haircut in the market so you can wait and hope it comes back.I am not a SMF apologists, I think he makes the fool of himself more than not. But in this case he is not wrong. It is a legit strategy that many employ. Others employ your strategy too. There is no right or wrong just more conservative vs aggressive.
You are also making a lot of assumptions in your take. Who is to say the stock/fund didn't double/ triple/ 10x in a year on top of any dividend it paid? Grant it, they could drop too.
As for the liquid part you keep mentioning, they are both liquid. Not sure what you mean as a person could sell their shares and pay off the loan at anypoint.
The last person on this board I'd take financial advise from is SMF.