1. Invest in Bitcoin with your spare money:
I can say that this advice is not necessary in today's bitcoin world. This advice was only relevant like a decade ago when bitcoin was been seen with alot of skepticism. Some saw bitcoin as gambling, some saw it as a ponze scheme while others saw it as a bubble the will burst.
Have you forgotten that we do have bear markets that last a while?
Investing with spare money means in case of a bear market, one won't be forced to sell of there bitcoins at a lower market price just because they have to cover other expenses. Same applies to one investing what they can afford to lose. Nothing is obsolete
Certainly, the first point is solid advice: you shouldn't invest, not only in Bitcoin but in general, if you don't have the budget to do so. Investing shouldn't put you under financial strain. If you can't afford it, you're better off not taking a risk you can't afford.
The second point is kind of pointless to be bonest, at least that's how I see it. Any amount of money lost would stink, large or small. A loss is a loss. I can afford to lose $2,000, for instance, but that doesn't mean that I wouldn't be mad or disappointed. Moreover, even if we're talking about Bitcoin, gains are still not guaranteed.