It's only been known for close to a century. It is the reason why standardized tests were accepted in the first place. It has been confirmed in many ways since. If they were ever ignorant of this fact, it is because they were willfully choosing to be ignorant of it.
Research also found that IQ tests are a better predictor of job performance than other available measures, such as interviews. This was first established in the military, then in the 50s and 60s was confirmed for a variety of jobs. Unfortunately the case Griggs v. Duke Power Co. make it illegal to use IQ tests, because it results in hiring fewer blacks.
But IQ tests can still be done indirectly. For example coding boot camps can use what is essentially an IQ test to decide who will be a student, and then graduation from said boot camp serves as a signal to companies that this inexperienced person is smart, motivated, and willing to learn. That signal is likely to be more valuable to the company than the training itself.