This is simply untrue. I'm not a gunner, nor do I have any desire to justify my own path. If anyone is trying to justify their path, it's you. I was a Biology major and then I went to medical school, what's there to justify? I would like to point out that having an advantage doesn't guarantee you anything. Also, you have no idea where I or anyone else who has commented on this thread is on the curve.
No one is saying that you will automatically do better if you have a science background, but you are most certainly at an advantage, particularly in first year, where a lot of what you're being presented is basic science. Also, no one is saying that you won't do "fine" with another background. You can do extremely well with any background as long as you put in the work. That's something everyone can agree upon, but don't tell me that having taken two advanced level cell biology classes doesn't put me at an advantage when taking cell biology in medical school. Or the three semesters of physiology/anatomy doesn't help me in those classes. It's an advantage, look up the definition. It doesn't necessarily imply a handicap for anyone else, nor does it guarantee success, but it helps, especially in light of the volume of material in medical school.
How significant of an advantage, well that's up for debate.