Bitcoin investment is not guaranteed because nobody knows what will happen in future. It's good you tell people the fact of what they're investing into so that they can invest with caution and with the mindset that anything can happen. Why they do say that it's not guarantee so that you don't use all your life savings to invest in bitcoin hoping for big profits, but invest only with an amount of money that you can afford to lose overtime, so that if the price goes against you in the future, losing 100% of your bitcoin portfolio, you wouldn't lose hope in life and become frustrated because you have created other means of income and not putting all your hope on your bitcoin investment.
[Ediited out]
Trading is not an investment and more risky than bitcoin.
Makes sense to me, the fact that Bitcoin is less riskier than investing in shitcoins doesn't mean that one should put all their funds into Bitcoin, just like you said, the future is unknown and to be on a safer side, it's best for one to invest what they can afford to spare, maybe 10-20% of their income into it, though Bitcoin has proven to be very profitable in the past and has added lots of value in the past years, we can only speculate the price and hope for a better future, it's not as if success is guaranteed that's why we still have to thread with caution.
Also, I'm not in support of trading and never, though some people have succeed in it, it's their choice and luck therefore I won't advice anyone to risk their money into something that based on luck, atleast Bitcoin has proven in the past years to be very trustworthy even though the future is not guaranteed moreover I've not heard much or any complaints from people concerning holding Bitcoin for a longer period compared to the complaints from traders who lose money daily, one can only lose money on Bitcoin investment if done the wrong way, like taking short-term profits or exposing their portfolio to hackers therefore it's less riskier and better than trading.