Perhaps that concern is eliminated with good test programs? As an extreme example, if I wanted to compare how different languages do array slicing it's meaningless to force python to implement array slicing without the [:] operator. But then this whole test program is meaningless anyway.

You're right that there's a lot of nuance here, and perhaps there's no better solution. It's not even clear to me if there's a meaningful question to be answered. "This ocean's warmer than that one? What part? In summer or winter? Did you measure near a blue whale's fart?"