By the time you retire you have more capital accruing interest, and generally more to invest in the attendant better investment opportunities available. That's what they meant.
My guess would be that there are very few people out there, at any stage in their career besides retirement, for whom interest and capital gains are more than a tiny fraction of their income. For most workers, things like salary, bonus, and (for a lucky few) stock grants make up 90% or even 99% of our income until we retire.