You seems to get him wrongly, he never said that we can't Invest into bitcoin if we don't have income source. Rather, he was referring to those guys who think that having a stable source of income is a must before a guy will think of getting started. Perhaps, even if maybe he said that a guy who does not have no income source as you said should not Invest into bitcoin, I would say that he's right because having no income source means that a guy does not have any means of getting money accept the one they give as charity it could be from our relatives or our love ones as the case may be, so you don't expect this set of people to be investing into bitcoin when they're supposed to use the money to get thier basic needs, but this isn't what he meant.
You can not or should never use all your income for investment that is risky and investment can bring loss, not profit. So if you use all money for investment, you can lose very big if your investment portfolio goes wrong. It's clearly to realize while there is another thing that is not aware by many investors. If they don't manage finance well such as using all income for investment, they will soon have to use part of their bitcoin for spendings.
This is very terrible practice and experience if you used $1,000 to purchase bitcoin and months later you have to sell it for $500 when Bitcoin corrected 50%. This scenario can be possible in a bear market and if any investor does not have well plan for their finance in bear market, they will sell with loss.