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People have posted books on arxiv for at least a decade. Their rules state: "Submissions to arXiv should be topical and refereeable scientific contributions that follow accepted standards of scholarly communication."

As I understand it, submissions are moderated (albeit lightly -- not to the degree of formal peer review). So, since it's on the arXiv, we can assume that the moderators felt it belongs there.

I've seen a number of other book-length items on the arXiv before, including e.g. Seven Sketches in Compositionality [1], so this isn't a particularly new trend, either.

[1] https://arxiv.org/abs/1803.05316v3

To post there you simply need someone to vet you. Typically your advisor, coauthor, or a colleague but they don't check the relationship. The system is rarely abused tbh. Introductions, surveys, and books are actively welcomed. At least I welcome them and find them extremely useful. We want open access (and open source) knowledge, right?

fwiw, arxiv has posted book-length works since it started in the early ‘90s.