Isn't the SAT a de facto IQ tests for Higher Education Institutions?

No not really. SATs measure a person's knowledge which an answerer on here already pointed out whereas IQ tests measure a person's intelligence. SATs are used to determine a person's best career paths based on how much knowledge they know from school. IQ tests are used to predict a person's potential mental acuity. A person with a high IQ could have dropped out of school and not have learned a lot of things but have a high IQ nonetheless. People with high IQs are fast learners who imbibe abstract concepts easily and effortlessly if given the chance. My dad dropped out in the 8th grade and probably wouldn't have got a good high SAT score as a result. However, he had an IQ of 145 and could easily learn new things. He taught himself useful hobbies in short periods of time. For example, in his retirement years he decided to take up painting as a hobby and learned it at a professional level within only about a month and a half. If he wanted to, he could easily study and learn all the school work he missed during his teenage years. 

in effect, it tests for your command of English, various maths, spacial reasoning, and logic.  Plus some high school prerequisite knowledge

The SAT and ACT test a student’s knowledge and how well they can use that knowledge. 

An IQ exam serve as a measure of an individual's intellectual abilities and potential

Take a read through this

https://blog.collegevine.com/do-sat-scores-correla...

Yes, pretty much.