They are reselling inference. That is not a novel concept, indeed, a stationary merchant like Walmart does conceptually the same. Although since inference doesn't require any stockkeeping, I would rather compare it to Amazon Marketplace.

But it's been novel for inference services when OpenRouter was founded. And nowadays they have a considerable foothold in the market. There are competitors, but the one everyone knows is OpenRouter.

And since they do not need to maintain any stock, don't need to do anything with queries but route them through 1:1, the most complex thing they do from a business perspective is that they collect your money and route the right amounts to various inference providers that served you.