(underlining is mine)
P2801.7 Water heater seismic bracing.
In Seismic Design Categories D
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, D1 and D2 and townhouses in Seismic Design Category C, water heaters shall be anchored or strapped in the upper one-third and in the lower one-third of the appliance to resist a horizontal force equal to one-third of the operating weight of the water heater, acting in any horizontal direction, or in accordance with the appliance manufacturer’s recommendations.
"or in accordance with the appliance manufacturer’s recommendations."
Thus, no strapping is needed if the manufacturer addresses "anchoring" in their installation instructions, and they should.
It's the weight of the water in the storage tank which is to be considered, not the weight of the appliance itself, and, tankless water heaters do not have a storage tank to hold water.