heating a fairly standard 120l tank of water from cold takes around 7kWh of heat, so heating that water will use 7kWh.
The point that I think would have been being made originally with that sort of statement refers to the systems where the hot water circuit was effectively always on while the heating system was on. In this case, once the tank is up to temperature it's not going to be extracting any additional heat from the system, so the statement is essentially correct at that point, other than pipe losses, which the old argument went weren't an issue as the heat still went into the house anyway.... nowadays many would question the real usefulness of heat from pipework being dumped into the void between floor and ceiling, but there's still a reasonable degree of logic to it.
It is true though that water heating from gas is much more efficient in winter when the boiler is already operating, the pipework is warm and the boilers not cycling etc vs using it in summer where pipework losses are significant as that extra lost heat isn't useful heat, the boiler will often be cycling, and it's got to heat all the fluid in the pipework each time it cycles.
Immersun etc work the most in summer when there's no argument at all about the heat replacement, and in winter could be viewed more as being used to keep the hot water cylinder at temperature between boiler cycles to prevent the boiler switching on and off just to warm the cylinder by a few degrees every hour or 2 (depending on the control settings for the boiler).
