I haven’t read the book. But I don’t think stock market corresponds to the law of gravity; if anything, it relates to the law of inflation and law of human evolution.
- Law of inflation: prices will go up. As prices go up --> companies revenues go up (maybe profits go up) --> stock market price go up.
- Law of human evolution (or Darwin’s survival of the fittest): in general, the human race evolves for the better, becoming healthier, smarter, more innovative, etc. With more innovations, that can only drive up the stock market.
Law of gravity has nothing to do with stock market, that law is only important when dealing with gravity - per Sir Isaac Newton. 
Notice I said “stock market”, not “stock” or “company”. I think target date funds are fine as long as they’re low costs. Also, I think some companies only offer target date funds as investment options in 401k.
Also, for the amount in high yield savings account, I wouldn’t put in it more than what is needed for an emergency fund. If you put more, that’s the mistake of leaving money on the table too long.
With all that said, yes, the stock market can go down in the short term. Be prepared to be comfortable with the losses you see during that time. Investing is only for those with long term mindset.
