This is a common misconception. People without academic affiliation (based on their email address) require someone to vouch for them before they can submit to arxiv. And papers submitted to arxiv (with or without affiliation) are reviewed, and many are rejected.

Papers on arxiv are only reviewed for formal requirements. They don't review every pdf there, and reject them for being false or wrong.

You are right that arxiv is an invite-only website, but once you are in, there is no peer review of any form.

arXiv does not review everything pushed to the site.

It's very easy to get in. It's becoming a common target for grifters who will "publish" papers on arXiv because it looks formal to those who don't know any better.