You're spot on - the calculations in the post are totally wrong. For instance, a Dice coefficient is itself the average of hundreds of thousands of observations (each pixel in an image), so you can't just treat the coefficient as a single data point. However you also can't just take a standard error, since the points are highly spatially correlated.
The post also doesn't draw on any actual reported experiences of either competitors or hosts of competitions. Generally in post competition wrap-ups there's a lot of information sharing and very significant development of understanding that comes about.
To take the imagenet example that's claimed in the post as being a likely example of over-fitting: each year's imagenet results led to very significant advances in our understanding of image classification, which are today very widely used in industry and research.
The post also doesn't draw on any actual reported experiences of either competitors or hosts of competitions. Generally in post competition wrap-ups there's a lot of information sharing and very significant development of understanding that comes about.
To take the imagenet example that's claimed in the post as being a likely example of over-fitting: each year's imagenet results led to very significant advances in our understanding of image classification, which are today very widely used in industry and research.