IQ is not the same thing as intelligence.
It is a particular slice of intelligence. And a malleable slice.
Take somebody with a 120 IQ and somebody with a 90 IQ, train both to make a table and the 90 IQ person may consistently make a better table. Train both to give a presentation and the 90 IQ person may consistently do better. Give both a sales job and the 90 IQ person may consistently sell more.
Intelligence is many things.
IQ measures a bit of it.
You are conflating general intelligence with specific skills. IQ is a measure of general intelligence. Building a quality table is a specific skill that need not depend on any kind of intelligence any more than the skill of running fast depends upon intelligence. Figuring out a novel method for superior table building without having lots of prior experience is something that depends upon intelligence, and the person with a 120 IQ is more likely to do it than the person with a 90 IQ. Now, if you give the 90 IQ person 10 extra years of carpentry experience, then they might be more likely to come up with a novel method. However, that isn't because intelligence is "malleable". The experience doesn't increase the intelligence of the 90 IQ person, but rather makes their intelligence less relevant to the task by giving them task specific knowledge that plays a larger role.
The performance of any task is the result of the interaction among several different types of factors, including general intelligence, specific abilities (cognitive and physical), experience-based training and knowledge acquisition, motivation, etc.. The particular tasks vary in how much each of these plays a role, and having more of one can make up for having less of others. Thus, general intelligence has its largest impact on tasks where the person has minimal prior experience and where a specific skill is not sufficient for good performance.
Take a 120 and a 90 IQ person that are equal on all these other factors, and the 120 person will do better on most tasks where general intelligence is relevant. The 90 IQ person will do better on tasks where the other factors have high relevance and they have more of those other factors than the 120 IQ person.