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I think the third possibility is just mis-communication. When they say vacuum retard, they mean retard from the vacuum mechanism. I say this because there is no way to get both vacuum advance and vacuum retard from the same port, on the distributor diaphram, as long as the source is also the same at the throttle body. You can get retard from boost pressure on the same port though, which is what I think they are really saying. The note says:
"The charge air pressure is used for full throttle retard ignition control, to avoid coming into the range of excessive advance ignition at full throttle with the ignition timing which is 5 degrees earlier than the USA version."
I think this statement refers to the Euro cars, which would confirm that the vacuum retard is really from boost.
With regard to ignition controllers with timing adjustment, keep in mind that they are adding or subtracting to the centrifugal advance that will still be in the distributor when the vacuum lines are removed, and that intake manifold temperature has a BIG effect on detonation limit, so stay conservative until you can drive your vehicle on a hot day to make sure that you are safe.
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