The original assertion was not "If you've done very well at a series of interviews and proved your ability already, and then they want to give you an IQ test, run."
Don't get me wrong, I love to see open source work, it is a plus and it definitely helps paint the picture of what the candidate is passionate about. But just having impressive open source work alone is not enough to make the technical interview unnecessary.
> and ace the technical interviews
If there's an IQ test after they ace the technical interviews, then that's a really weird order to do the interview, but I don't think that's the situation that the above comments were talking about.
Open source is usually a free time pursuit, so quite possibly they did spend several days on a small change because they had better things to do. Or the small change was the result of several days of tracking down a difficult bug.
My point was however that whether or not IQ tests are effective, making experienced developers sit through them (or indeed junior developers who may have verifiable academic or other accomplishments) could be seen as insulting or degrading. If your goal is not to unnecessarily piss off good candidates who may then bad-mouth you to their friends or bring up the issue on Glassdoor, then IQ tests - even discarding the legal implications - are perhaps not such a great idea.
Let's say you're an experienced developer. You have an impressive work record, open source contributions, and ace the technical interviews.
Then your potential employer asks you to take an IQ test.
Legality aside (and it may be framed as a "general aptitude test" or somesuch), this may be very insulting to some people, who feel their contributions and achievements should stand on their own without bullshit tests. It signals to them "we don't care what you've done, and you can prove you can do, or your years of experience or education, you getting the job depends on an arbitrary test we're going to throw at you". You're not going to be treated as an individual, but rather a set of points on a score sheet.
Now you may be right or wrong as to whether these tests are indicative of ability, but for a lot of applicants they would be a red flag.