I'm sure you're technically correct that the standard isn't truly "0", as that doesn't really exist in an analytical testing sense, but the larger picture is that any use/exposure puts a person at risk of failing. As indicated in that link, even the use of widely available CBD products has resulted in failures, which can be career ending as well as the cause for pretty dire penalties. There are also positions where extensive investigations are conducted beyond just the standard panel, including multiple interviews, contacting friends/family/neighbors etc, potentially a polygraph, all of which cover substance use. So yeah, the testing protocol doesn't have "0" as the actual standard, but the DoD policy is effectively 0.