The point is like this, having a stable income does not guarantee that someone will have discretionary income. Because it could be that even though the income is stable, the nominal amount is not too large and is only enough for daily needs (so the point is not having leftover money or discretionary funds). So having a stable income also cannot guarantee that someone can invest in bitcoin. But if people have discretionary income, it is certain that the person can invest in bitcoin and it can also be concluded that the person's income is quite good, because they have leftover money. So which of these two assumptions is correct, in my opinion the most appropriate is that people who have discretionary income can invest in bitcoin safely. Because the point is the important point is in their discretionary income, when it comes to income, it's up to you, whether it's stable or not. Because there are quite a few people who have unstable incomes, but when they have income, the amount of money they get is very large.
The important thing is not necessarily that your income is steady or not but whether there is something left over after taking care of your essentials. There are those who have monthly income yet still find themselves caught in a pay check to pay check lifestyle and there are also those who have an irregular income yet they are able to save and invest wisely. Well it would be best to wait till you can have some more space in your budget to invest in Bitcoin, than to endure basics by straining. Low coiner is better than no coiner provided you are consistent and smart about it.
I want you to understand that when we say income is steady it means that it is not constant and so an investor doesn't need a steady source of income but rather a stable source of income because of stability. When someone has a stable source of income and they are determined to invest in Bitcoin, they will definitely grow there portfolio more than one can imagine and just like you said any investor who is doing well without a regular source of income ( without a stable source of income), if they are opportune or privilege to have a stable source of income they will wonders in there investment.