Passive income costs an enormous amount of active life up-front. That's not a warning. It's just the receipt.

The thought behind it

What looks free is being paid for somewhere. The sentence points at where, and declines to soften it. In The Long Game the reward is real but late, and the only skill on offer is staying. Everything hinges on “up-front”, which is the part a politer version of this sentence would have left out.