Not sure about the original Japanese but assuming this were really said in an English context, I believe it's just an expression (at least this is how I'll give the principle of charity I guess). Of course there's all this Carol Dweck stuff of fixed mindset vs growth mindsets aaaaand:
IQ is largely a pseudoscientific swindle by Nassim Nicholas Taleb
The learning myth: Why I'll never tell my son he's smart by Salman Khan
Josh Waitzkin interview:
In general, do you see any disadvantages to being labeled a child prodigy?
Yes, there are huge disadvantages if you buy into the label. The most perilous danger, in the language of Carol Dweck, is that we internalize an entity theory of intelligence. The moment we believe that success is determined by an ingrained level of ability as opposed to resilience and hard work, we will be brittle in the face of adversity. For that reason, it is incredibly important for parents to make their feedback process related as opposed to praising or criticizing talent. Think about it-if you tell a kid that she is a winner, which a lot of well-intentioned parents do, then she learns that her winning is because of something ingrained in her. But if we win because we are a winner, then when we lose it must make us a loser.
But it's not like 'IQ' is a complete swindle in a heuristic sense: Fuutarou's been studying hard much earlier than they have, so Fuutarou has
better study habits and
is able to pick up new topics with less difficulty because e has a solid foundation of the prerequisites. Eg If you didn't study geometry so well, then you may struggle with trigonometry.
- In this specific case...they were most likely studying English or Japanese (Fuutarou talks about 'author'), neither of which are high in prerequisites compared to maths or science, so maybe it's not this.
So, 'IQ' here then could refer to either
Fuutarou's good study habits of, say, not going to a hanabi festival without completing homework
Fuutarou's good foundation in, say, geometry
Finally, additionally, Fuutarou just likes studying while the quints don't really, so 'IQ' here then could refer to 'desire to learn'.