There were no restrictions experienced in the older models of water heaters. What the problem was they did permit solids/particles to gradually deposit at the tank bottom. When sufficient depth of this sludge was present, the effectiveness of the lower electrical heating element or the gas burner which is only at the bottom of the holding tank was interfered with. The even flow of water not being in contact with the heating element caused it to overheat & reduced & compromised it's useful life. The heating element had to produce more heat to achieve the called for water temperature.
Well there is no logic such as that hot water can be restriction in the water flow so you need not to worry about this. You are right your perception about water flow is right. There is no such thing as resistance in water circulation.