I assume the meeting of the "number" stated here is an estimate of how much wealth would need to be invested in a stock portfolio to yield 1500/month net of taxes and inflation, without needing to trade time/labour for income.

Pretty reasonable, but without knowing more about OP's situation, advising someone asking how they can earn more income on the side to earn it by having $1m wealth invested in the share market may not be the most immediately actionable advice! A bit catch 22 if the current problem is "not enough surplus income".