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An electronic wastegate reduces the amount of engine power being sent to the turbine which will just be wasted on spinning a compressor at part throttle.
If you have a turbine that minimizes backpressure and use an electric motor to give the compressor a boost, then if you decouple the compressor you can extract a small amount of free electrical power even at part throttle.
The principle is the same: Don't send power to the compressor if you don't need it. You can think of the power in the second case as power from the battery. In the first case, it's power supplied via backpressure.
In practice, an engine isn't producing much blowdown energy at part throttle, so there's not much to collect, but hypothetically, a hypermiler would be better off deleting the compressor for efficiency.
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