Hence GP says 'diversify'. If you weren't an employee, or were one with no related stock in the company, would you buy so much, invest ~everything, in that (or any) one company? No? Well, don't treat holding differently to buying. It's the same, modulo fees & tax.

My econ teacher taught me nothing about personal finance. Is this normal?

I dunno, got taught in Europe, got micro, meso and macro in different years, and sometimes semesters. My econ teacher was also a bit weird in a good way, a real character with a lot of knowledge. Ofcourse I didn’t do anything with it, wish I had.

Michael Saylor says only invest the money you can't afford to lose.

This is likely closer to reality, if one is cryptocurrency savvy.

I don’t really get this point… how does that work?

If you can afford to lose your money, don't invest it into cryptocurrency and keep it as USD or other fiat.

All the ghost industries like US airlines kept afloat by money printer go bur. It keep going and the intrinsic value of a BTC go up. (Numba go up)