While I can't say if this was what the author was thinking.... I can play devils advocate and propose a way an idiot with this kind of ability MIGHT still somehow think he was weak....
[Damage Resistance]
If you've never heard those words before.... just follow along with this RPG inspired explanation.
Say we have a target with [Damage Resistance 10] (DR10), and just to give it a number lets say it has 100HP. The DR10 means that any individual attack will have its Damage reduced by 10 before it can chip away at that precious HP. If we assume the average punch does 2 damage, then it doesn't matter how many times we punch it cause any single attack has to do 11 damage or more before DR to reduce the Target's HP. (2 Damage - 10DR = 0 HP Lost. 15 Damage - 10DR = 5 HP Lost)
Now lets assume our our basic [Fire] does about 3 damage. Even if we can infinitely cast [Fire] it doesn't matter, since [Fire] can't deal any damage to the target.
If our idio-
If our protagonist came to the conclusion that layering thousands of [Fire] spells only meant the Target would get hit with all of those individual attacks at once, then DR would still dictate each of those attacks gets reduced to 0 for a total of 0 damage.
"Even if I hit it with [Fire] 10,000 times in a single second, it won't even leave a mark!" Again, yeah, if each of those [Fire] spells does their damage separately then this thought would be sensible.
.....But everyone's reaction in the chapter should key us (the readers) in on the fact that layering isn't ONLY making a lot of tiny spells hit all once...... it is effectively making ONE BIG SPELL.
So instead of doing [3 - 10DR = 0] * 10,000 Damage (0 Damage)....... hes actually doing [3 * 10,000 ] - 10DR Damage (29,990 Damage)
Bakato-kun here really should have done a better job of testing how layering his magic worked....
But ultimately this is more likely just poor writing instead of being a play on misunderstanding how his damage was getting calculated.
