You are playing in the margins there. Very few people have an IQ of 80 or 180.
The point is, no DNA test is going to tell you if a hard working person with a 100 IQ will be more successful than a lazy person with a 115 IQ. Similarly, no test will tell whether a sober person with a 110 IQ will be more successful than a drunk with a 125 IQ.
Actually, there are a lot of people with IQs of ~80 (or lower). Roughly 15% of the US population, in fact. There are not many at 180, though, that is for certain. And yes, an IQ test result of 110 or 125 can't guarantee performance (although your example is smaller % of population than my 80 IQ), it massively increases the odds - but those are still for relatively close IQs in the scheme of things. How about a 95 IQ (just a hair below average) vs a 125? The person with 95 IQ will need to work probably 3-5x as hard to get a phd as someone with 125.
I actually did that test once.
I took an IQ test sober, then had four drinks and took a very similar IQ test.
My IQ as expected, went down by 20 points. I was surprised the decrease was that small.