Perhaps for performance on a fixed kind of test or for certain kinds of raw calculation speed, it is true, but handling complexity and problem solving skills can continue to develop based on experience and exposure to concepts.
The belief that intelligence is fixed is a trap for the mind.
It's a morass I'm uninterested in getting into.
I agree your original comment used the conditional ‘if’ about the truth of fixed intelligence.
Such a proposition naturally invites examination of whether the conditions are in supported by evidence, which is all I am doing.
I am curious if you have a definition of intelligence that sheds light on your original argument.
I have no stake in what truth you believe.
However I think your argument is clearly incorrect.
As you suggest, I could be basing my conclusion on a different definition to the one you are using.
But if you won’t produce a clarifying definition, you can’t reasonably make claim to be operating under any principle of cooperation.
I think it’s very likely environmental factors and personal mindset have considerable impacts both positive and negative on intelligence. I also suspect in the next decade or two research will come out on pharmaceuticals that show improved mental performance. There’s already the beginnings of this.