Looks like the default config in the chat is instant 5.3, it only uses the 5.5 on the thinking variant
They moved a few months ago to have separate instant and thinking models. 5.3 is the latest instant, and 5.5 is a reasoning model.
alt Hacker News
They moved a few months ago to have separate instant and thinking models. 5.3 is the latest instant, and 5.5 is a reasoning model.