Does “Autostart” Mean “Autostart”?

Nothing surprising. Many (misbehaving) programs did check on every start whether they are still registered as startup program an just set the registry key again if not. So instead of removing the key when disabling it in Msconfig, Microsoft set a "disabled" key somewhere else so Explorer would ignore them.

Sysinternals did not have the luxury of modifying Windows Explorer and so just moved the key to somewhere else, with the disadvantage that the misbehaving program would just restore the registry entry at the original location.

If there are mechanisms for autostart and disabling autostart, isn't there, like, official documentation for that?

It's clear that Microsoft won't listen if a few casual home users demand documentation, but I would have expected huge institutional customers that throw two or three digit millions of dollar per year at Microsoft to demand and to get documentation for such things.

In 7 (I think) an option was added to task manager to temporarily disable or enable autostart entries. I suppose that this is how it is implemented?