I would agree with this, good explanations and rigour are definitely not mutually exclusive. But I am not sure what you are expecting. This is a research paper, intended to be read by other experts. Even still, if we look at how the paper is organised, the authors start with a high level overview, and build upon simpler problems to leverage their way to an understanding of the main result. For a research paper, it is extremely thorough.

If you want a 'simple explanation' that allows a layman to gain some understand the problem, sure. But this isn't, and absolutely should not be, the purpose or content of the research publication, which is to succinctly communicate to other experts the results of their findings.

simple != simple for laymen. It doesn’t make sense to recap the entire history of a field in the intro

It's hard enough to create the thing and make your explanation fit on the page limit. You also want people to make it comprehensible at the same time?

If you want to, you can study it, make it easier to understand and write another paper. If it's good, you will do both yourself and the original author a huge favor.