Auto-translated
If it's not clear yet, the problem isn't about cutting down 40 demons, but about doing it with minimal loss of tempo.
You're right, cutting them down isn't a problem, it's just that more important goals often appear, like grabbing something, dealing with a night stalker around the corner, or many other things, and then the efficiency of the cutting won't be 100%, which is often the case...
Both sides are right in their own way, I think every self-respecting Heroes player should be able to produce forty or fifty units by level 117, even in single-player.