So I took the quote to mean "among the gifted, an early reader isn't necessarily more gifted than a gifted child that doesn't read early". Among that population, you can't pick the early readers from not by 4th grade.
I think it would have been less bad if she'd been saying that, but the quote about early reading as a milestone makes me think that she was talking about all kids.
Either way, your note about people believing that the statement applies universally is spot-on.
How come no one ever says this stuff about gifted athletes?
I actually agree that reading can be just a milestone, but certainly not that it always is (or even usually is!). This is why Ruf levels never make any sense to me because they depend so much on early milestones that are dependent themselves on oodles of inputs, many which seem only vaguely related to giftedness. I can't imagine *any* early milestone being universal for HG kids. I can't even find any "universal" in my own house with multiple different kids.

I accept that some kids aren't interested in reading, some may not have visual control developed enough to read early, and some may not have access to books and Sesame Street or Blue's Clues. Some kids like things like puzzles more and develop in ridiculously advanced ways but not necessarily text related.
But . . . . . for one of my early readers, others catching him by 4th grade cracks me up. He was junior high level in K, past high school by early 2nd grade and the gap widens daily. Another HG kid learned to read at 2 without instruction because he likes to crack codes. He learned time and money, math, etc., for the same reasons very, very early. He's a gifted learner, but I expect many MG kids will read better by junior high. He tests way above grade level, but so do lots of kids who learned to read at 5 or 6. DD learned to read in school and now reads considerably above grade level. I just don't see how early means better in walking or reading or math or anything. For some kids, it indicates some serious ability. In some kids, it's just speedy decoding or parental pushing or rich environment or great decoding.
I'd be inclined to think the same thing is true of athletes and thought that kind of thing was generally accepted. I'd guess most Olympic gymnasts walked before average age, but most kids who walk at 9 months don't end up to be Olympic gymnasts and others do catch up. That doesn't really mean much since it doesn't tell you that your kid walking at 7 months will be a fabulous gymnast any more than reading at 2 predicts a PhD or Nobel.
Kids seem to develop at different rates. I find where they end up far more interesting than how they start, and I suspect many late bloomers develop at astonishing rates once they get going. My kids learned to walk at different times but they all walk well now and the same is true for reading.