Like most 'Extensions' Fredo's tools usually have a Plugin entry which loads the Extension setup code and the Extension itself - with a similar name.
If you use the SCF Plugins Manager to Disable any Plugin, then it will not load when SketchUp restarts.
If that Plugin also happens to be related to setting up an Extension, then that Extension is not set up, and therefore it is not visible in the Extensions Manager at all.
However, if you Enable that Plugin, then it will load when SketchUp restarts; and the Extension will then be visible in the Extensions Manager - its 'state' will be as it was last time you used it [Enabled/Disabled]
So to reiterate...
All Plugins [.rb/.rbs scripts in the Plugins folder] can be 'Managed' [Disabled/Enabled/Temporarily-Loaded and Uninstalled].
Only an Enabled Plugin can set up its related Extension.
An Extension can be 'Managed' when it is set up: its last 'state' is remembered across sessions.
Because you can't dynamically 'unload' a Plugin or an Extension the effect of Managing them is not always obvious until you restart SketchUp. Enabling things is obvious, as the thing loads and changes color to 'green' in the left column to remind you that it is loaded and will reload on a restart too; but Disabling it will still leave the thing loaded, so its shown 'red' in the right column - to remind you that the change is only completed by a restart: and also a Temporary-Load from the right column leaves it in there, but colored 'orange' - again to remind you that it is currently loaded but it will not be loaded after a restart...