It’s my understanding that, while these processes can produce an atmosphere of mostly oxygen, it’d still be a much thinner atmosphere than ours, so a thick oxygen atmosphere would still catch our interest.
But it does illustrate a good point, in that that process wasn’t hypothesized until relatively recently, and so before then, if we’d found such a world, we’d have assumed life.
Now, it might be reasonable for Star Trek to have scanners that can detect “life signs”. The Federation has seen thousands of worlds up close, both life-bearing and otherwise. With that kind of data, you can start doing some serious science. We, meanwhile, have only sent probes to the surfaces of four worlds, and only one of those (to the best of our knowledge) has life, so we don’t really have any idea what the true, reliable signs of life are.