> ARR seems like a interesting concept

Yes, it is a meaningful financial measure of actual progress, exactly where progress is most needed for a new company.

> which hides away the concrete details

A normal metric isn't a magic trick. It's just the number it is.

> The crucial detail is probably the money being spent

Accelerating (not just fast) revenue growth at an astounding rate, with absolute numbers that are enormous for a new company, bonkers for a 2500 employee startup [0], is the crucial "detail".

There are lots of companies with deep pockets and compute, making great AI efforts with teams of smart people, that would love to be doing, but are not doing, what Anthropic is doing and doing well. That might be the second crucial detail.

[0] https://en.wikipedia.org/wiki/Anthropic

> A normal metric isn't a magic trick. It's just the number it is.

ARR as a number for steady business or public company is much simpler, ARR for companies at this early stage fueled by the motivation for IPO is more than just a number. Its an attempt to convince the market for a certain outcome without revealing the books

> Accelerating (not just fast) revenue growth at an astounding rate

This acceleration is fueled by somewhat artificial demand (AI mandates by executives across the board, still figuring out the realistic use cases) and subsidized pricing. The expenses matter cause they are the ones which are going to dictate if the business is sustainable or not.