Discretionary income must not come from your job salary directly, there may be more income sources apart from your direct job. I have friends that work a full-time job while being an agent in other people's businesses and it's possible they use what they get from their salary as expenses and their referral bonuses as discretionary income and if they're into Bitcoin, they could still use the referral bonuses to invest.
Again my youngest sister still insists I send some money for her at the end of the month for her upkeep despite working herself and making money on her own which I still do and it's possible that the money I send to her forms a large party of her discretionary income.
Discretionary income is basically any income that you've left after catering for your expenses and it doesn't matter the source, what matters is that you recorded it as an inflow and you've handled your expenses independent of it and such extra funds can be used to invest into Bitcoin.
I don't know why some people are still finding it complicated, what you are saying here are true, discretionary income can come from anywhere, as long as it's the money left after all basic needs has been met.
The only problem about most Bitcoin investors is that they still can't figure out their discretionary income, they invest outside their discretionary income feeling like they can easily replaced the money used to invest from another source, and mostly times they will be unable to do it, which might get them into trouble with their investment, they don't know that it's only the rich guys that has such leverage when investing in Bitcoin, and it's still only the rich guys that can invest in Bitcoin without an emergency funds and go Scott free with it because they have numerous source of income and they also have rich friend and uncles to cover for them when in real life emergency, but if you are not in such a level, don't ever try that shit because your bitcoin investment will be sacrifice when real life emergency struck.
So investing only from your discretionary income and having an emergency fund in place while investing is very much fundamental to the security and safety of your bitcoin holdings.