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.