I'm no bio researcher but benchmarking 0.4% salt in diet and 8% salt in diet sounds more like an experiment in testing an extreme, unrealistic edge case rather than anything meaningful.

I mean even 0.4% sounds like a relatively large amount of salt

Lacto fermentations like 2-3% salt by total mass. So if you ate nothing but pickles and drank water, you might have around a 2% salt diet.

Then again I wonder about pizza, hotdogs, and potato chips. They're crazy salty, but don't underestimate how salty 3% really is.

This rat study used an absurdly high salt concentration; no human food is anywhere close to that, and if a human did try to eat that much salt, they would die of dehydration or damage their kidneys.

But, predictably, this study will be used to generate headlines implying that salt is bad for humans, in ordinary contexts. Paste the paper title into Google News, and there they are. I believe this was likely the paper authors' intent, and that this is a high-prestige form of pseudoscience.