[NilsIRL]

FLoC has a well defined API like (all) standards: https://wicg.github.io/floc/

There's no reason why the website would not be reliable at detecting whether FLoC is enabled on your browser.

You can also used https://floc.glitch.me/ which was linked in one of the blog posts from google.

[stefan_]

In the sense that you can call "side channel for ill-specified data Google wants to leak" an "API".