is calling something “arguable” really an assertion?

Yes. “Arguably unicorns exist” is me asserting there exist legitimate arguments for unicorns existing. By saying it this way I get to imply they exist without having to provide any real arguments myself, and if anyone attacks my position I can fall back to “I’m not saying they do exist, just implying there are other people who think they exist without providing any sources or arguments actually supporting that claim.”