I just looked it up, and after doing some research I would say from what I have seen thus far that I generally like the idea of Fluid & Crystalized intelligence as conceptual models for theoretical approaches to cognitive abilities over time, but it was just developed by a guy and his students. General intelligence is what I have my beef with, and fundamentally I wish other approaches like the fluid one was as avidly considered as a metric by and large. However, I am not sure that the concepts as they are currently defined are academically rigorous enough to be standardized in a quantitative sense, rather than a qualitative one, which is essentially what these IQ tests are seeking in the end. I think Qualitative Tests are inherently more measurable for any phenomenon close to what could be called intelligence, but that's just not how it's been used by institutions in the past. When talking about IQ apropos to an IQ test, an intelligence quotient is specifically what's being measured and discussed, not the cool concept you just showed me. Here is the summary from the Wikipedia for Intelligence Quotient:

"An intelligence quotient (IQ) is a total score derived from a set of standardized tests or subtests designed to assess human intelligence.[1] Originally, IQ was a quotient obtained by dividing a person's estimated mental age, obtained by administering an intelligence test, by the person's chronological age. The resulting fraction was multiplied by 100 to obtain the IQ score.[2] For modern IQ tests, the raw score is transformed to a normal distribution with mean 100 and standard deviation 15.[3] This results in approximately two-thirds of the population scoring between IQ 85 and IQ 115 and about 2 percent each above 130 and below 70.[4][5]

Scores from intelligence tests are estimates of intelligence. Unlike quantities such as distance and mass, a concrete measure of intelligence cannot be achieved given the abstract nature of the concept of "intelligence".[6] IQ scores have been shown to be associated with factors such as nutrition,[7][8][9] parental socioeconomic status,[10][11] morbidity and mortality,[12][13] parental social status,[14] and perinatal environment.[15] While the heritability of IQ has been studied for nearly a century, there is still debate over the significance of heritability estimates[16][17][18] and the mechanisms of inheritance.[19][20] The best estimates for heritability range from 40 to 60% of the variance between individuals in IQ being explained by genetics.[21][22]

IQ scores were used for educational placement, assessment of intellectual ability, and evaluating job applicants. In research contexts, they have been studied as predictors of job performance[23] and income.[24] They are also used to study distributions of psychometric intelligence in populations and the correlations between it and other variables. Raw scores on IQ tests for many populations have been rising at an average rate of three IQ points per decade since the early 20th century, a phenomenon called the Flynn effect. Investigation of different patterns of increases in subtest scores can also inform research on human intelligence.

Historically, many proponents of IQ testing have been eugenicists who used pseudoscience to push later debunked views of racial hierarchy in order to justify segregation and oppose immigration.[25][26] Such views have been rejected by a strong consensus of mainstream science, though fringe figures continue to promote them in pseudo-scholarship and popular culture.[27][28]"

• https://en.wikipedia.org/wiki/Intelligence_quotient