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.