it's a number 2 years out of date. they invested 8b and ant is now worth 183b.

That doesn't tell us what percentage they own, though. When you invest in a company and the value goes up, your percentage doesn't change (unless there are additional investors)

that's really naive im afraid. you have to take pro rata or the percentage goes down. amzn did not take pro rata.

I am not sure what you mean by naive. I also didn't say the percentage wouldn't go down, I literally said other investment rounds would change the percentage they own.

Amazons investment in Anthropic was in the form of convertible notes, which they have converted entirely into equity by march of this year. At that time, Anthropic was valued at 61.5 billion and Amazon (in their filings) said their investment was worth 13.8 billion, so about 22% of the company.

Then, there was another round in September where Anthropic raised 13 billion more at a valuation of 183 billion (so the new investors are buying about a 7% stake in the company). Without more details, that would lower amazons percentage to about 20% (old investors hold 93% of the company, so Amazon's 22% of the remaining 93% comes out to about 20%). There are probably other details that lower that percentage a bit, but i think the 15-19% ownership estimate is pretty accurate.