I get your point but discretionary income isn't necessarily money you can afford to lose without it affecting you. It's just funds you could use anytime for anything without sourcing them elsewhere.
You don't necessarily need this discretionary income before starting, It is something you could build along with your investment.
Discretionary income should be money you can lose without affecting your day to day life or stressing you. It's not just about having money you can use anytime you want, it's about having a backup that doesn't affect your basic needs.
When you don't need a discretionary income to get started, I think it's risky, especially for those who are struggling to get their financial lives together. Imagine investing money you'll need next week for bills or food, and if the market drops, you will feel pressured and may make wrong decisions out of fear. That is why I believe it is better to set aside any small amount for investment. So you will not be confused in time of an emergency.
Sure, you can build discretionary income along with your investment, but the beginning is always important. Starting with a good strategy can increase your confidence and patience in the journey. For me, I'd rather build my investment gradually and maintain my peace of mind than rush in and get lost because I did not plan it properly. It's safe especially for newbies or low income earners, to invest with money that won't hurt them if things fail to go as expected.