For the variability point, they do tests in as a controlled environment as they could, and do the tests until they get consistent data. But what do you mean by significance test and how can they do it?

I agree they're not the gold standard but they're the best we got in terms of independent third party testers, and I would assume they're more than good enough for tech stuff.

Idk why they don't just run the test 3 times and average the result.

It would fix a lot of these issues... If you are testing a cooler, remove, clean CPU, reapply thermal compound and reseat cooler.

It's really not that hard to make their testing, maybe not perfect, but a lot better

I'm not really sure if you ever watched a GN video, but he throws facts at you at a rate which no one else i know does.

He knows his stuff and is always open to learn new things.