You are absolutely correct, it is not impossible for someone to invest in Bitcoin without a source of income I mean what they fuck are they using to accumulate if they don't have source of income.
Having a source of income and having a discretionary income are two different things you need to take not off, they are not the same, what is needed to invest in Bitcoin is a discretionary income not a source of income, because their are so many folks that have a source of income but still can't figure out their discretionary income, so if you have a source of income their no guarantee that such person will invest in Bitcoin and be successful if he can't sorts out his discretionary income from there.
Having a source of income is very important in taking care of yourself and your basic needs, but if you can't figure out your discretionary income and you went ahead and invest, you are just gambling, and the end product of such actions wouldn't be pleasing because selling it prematurely will be inevitable.
You need money to be able to invest, a source of income breeds discretionary income, so while a source of income does play a crucial role in generating discretionary income, you don't need a steady source of income, the periodicity and regularity of your income doesn't have to be steady, it just has to be enough for you to be able to get a discretionary income from it after taking care of yourself and your essentials.
So you need discretionary income to invest and you need a means to renew you discretionary income if you want your investment to be consistent but not a steady source of income.
Investing requires a steady flow of income. If you don't have a steady stream of income, where will you get your discretionary income from? When you are just starting out and learning about Bitcoin, you may not have a steady source of income. But to accumulate Bitcoin in future, you need to find a steady flow of discretionary income and deposit with it, otherwise you won't be able to stay invested for the long term.