Although I do not live by the source of income in btc but I think there are people living like that. A person who is good at investing and doing it in the short term can live up to the profit they receive. It sounds complicated, but you can understand there are people who rely on btc income and short-term investments to help them do so.
The line in bold letters is actually INACCURATE. Short-term investment is not actually profitable in the world of cryptocurrency, especially in bitcoin. Cryptocurrencies are known for their volatility, and you won't earn a lot if you are going to invest on it in a short period of time. Most people think that short-term investment is a good idea, but it is in fact a risky run. Example, the first recorded price of bitcoin in 2010 is $0.08. A man once bought a lot of bitcoins (that's a form of investment). After some time, he used 10,000 bitcoins to buy 2 pizzas. If he kept that bitcoins for long-term, he could have been a multi-millionaire now. With that, you can see that short term investment is just a waste of time and effort.
Also, there's no such thing as FIXED income in trading. Again, the price of cryptocurrencies are volatile and no one can say the exact value of these coins after some times. Bitcoin is also not a reliable investment, at least for now, because of its high price. If you want to invest in cryptocurrency, there is a lot of altcoins with high potentials that you can choose from. It is also not ideal to invest to bitcoin only (or to one altcoin). Diversifying your investments will give you better chance of earning.