Hi @mrainaldi,
Here's how I describe these related, but distinctly different features to instructors at my university.
"Hide Grades" is used to hide grades already released to students. Students may have already seen their grade in Canvas or received notifications including their grade info, so it's important to note that even after hiding grades, students may still be able to see what was done.
The posting policy is used to control what will happen when grade changes (modifications or new grades) are made. Setting the policy to manual will not hide already given grades, and similarly hiding given grades will not change the posting policy. They are two distinct things with different use cases brought up by teachers in the community over the years.
It sounds like you'd like to link these two features, and while that may work well for your use cases, I think there will be a lot of others who would not like that change. You could definitely submit this as a feature idea, to see whether Instructure would consider it.
-Chris