Anyways
It only presumes the first condition. The second condition is irrelevant for the effectiveness of a stimulus. A one-time stimulus only affects debt and growth in the short run (around 5 years tops), so long run trends (meaning longer than 5 years in this case) are not relevant.This presumes two conditions; 1) the growth created by the stimulus is larger than the debt it creates, and 2) the long-run GDP growth is greater than the growth of debt and debt service costs.
Talking about the effectiveness of a stimulus being affected by debt 10 to 50 years after the fact is silly.