The main reason why discretionary income is important if you are opting for DCA is that for DCA we need an amount that can be large or big but we have to make sure that this amount is available every week for investing in Bitcoin.
I don't think that the main reason why discretionary funds are important in terms of Bitcoin investment is not what you assumed. The main reason for having discretionary funds is to ensure that someone does not tamper with the money meant for essential needs just to invest in Bitcoin. This helps the investor avoid falling into a state of depression.Also, we don't need a large amount of money to start investing in Bitcoin. By using the DCA method to accumulate Bitcoin, as long as we have a source of income and maintain discretionary funds, we can consistently use part of those funds for our DCA strategy.