The point of the headline isn’t to communicate that their debt is currently rated junk, it’s to communicate that it’s impossible for their rating to improve any further without becoming investment grade.

From the article:

"The rating action, which has not previously been reported, shows Anthropic is close to achieving the investment-grade status that would allow the start-up to lower its borrowing costs and tap a wider pool of institutional investors including life insurance companies and pension funds.

Bankers for Anthropic and rival OpenAI have lobbied the largest rating agencies to grant the AI labs investment-grade ratings once they go public."