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So instead it pulls in hot water, but not for long enough to purge the line, so all the 140F water it pulls from the water heater sits in pipes till it cools off.

That’s pretty wild, since the energy needed to get it up to temp would still need to be expended on the water heater side. There are no real energy efficiency gains unless they can somehow engineer an effective cold water cleaning.

Many hot water heaters use gas which may be cheaper than resistive heating depending on where you live. Additionally, there are now water heaters that use a heat pump which will be more efficient than either.

And of course they'll be setups that use solar heating or are programmed to heat by time of day.

This is all down in the weeds though because a dishwasher does not use very much water.

there is typically only 1 program that is used for labeling (of overall efficiency to achieve appropriate cleanness level). my manual specifies which one it is. it takes 3 hours. there is also fast&furious program that is less efficient but takes 1 hour