I doubt anyone is saying that a steady income is not important, but instead saying that a steady income is not required in order to determine to invest into bitcoin. The thing that is needed is discretionary funds, and discretionary funds can come from a variety of places, including from non steady income sources, or even prior building up of back up funds or even from the use of debt... and yeah, even though a steady income can make things easier, it is not required in order to be able to buy bitcoin.. .what is needed is discretionary funds.
Many people believe that you can only set up a discretionary fund if you have a steady source of income, however stable income is not necessary before investing in Bitcoin. The main thing is to have some extra cash that you don't need to cover your other needs, this money can come from everywhere like side jobs, savings, or even some money you've saved for a long time, and there's no need to do this with your monthly salary.
Of course, if you have a steady income makes it easier to continue investing and building your Bitcoin over time with a rest of mind, but it is not necessary if you understand how Bitcoin works. And as long as you have spare money and you can afford to lose it without affecting your other expenses, you can invest in Bitcoin. What matters is that you use funds that you are willing to risks rather than relying on your monthly salary.