That is also my recollection.
Yep, that’s an advantage of having an intake manifold. Motorcycles don’t have a manifold for the most part.
If you have an intake manifold with a common plenum, then the vacuum will be the same on all the carbs no matter how un-synchronized the throttles are.
Motorcycles don’t have intake manifolds, each carb feeds its own cylinder with a few exceptions. The Yamaha V-Max has a valve between the intake runners that is closed at low speeds making it the normal one carb per cylinder setup, but opens at high rpm and throttle opening allowing each cylinder to breath through more than one carb. This allows smaller venturis to be used giving better low rpm throttle response and torque while still allowing high air flow at full throttle and high rpm.
Yamaha also had some one cylinder dirt bikes with four valve heads with separate carbs on the two intake ports and these carbs had staged throttles, at low speeds, only one port was feeding the cylinder, the secondary carb didn’t even have an idle circuit.
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Not with the device I showed in the picture. It mounts on top of the carb to measure the relative pressure drop across the restricted opening on top of the carb. Setting each carb to the same pressure drop syncs the carbs so each provides the same volume of idle air. Otherwise I agree with you. Bikes use individual runners so each vacuum signal is somewhat unique to each carb.