Water filled tanks used in this way (ie a body of non potable water with a coil passing through it containing potable water) are called thermal stores and are used in the domestic heating industry quite widely. The problem, when used with heat pumps, is that you really need either to heat the stored water at a temperature quite a lot higher than the desired dhw temp, or to have a much larger tank, to get adequate capacity.
From time to time there is discussion about this in the heat pump thread over on buildhub. With more modern heat pumps that can achieve flow temps of 70+C, and using a plate heat exchanger instead of a coil to heat the dhw, it might well be possible to make it work. Unfortunately the heat pump industry has pretty much adopted as a religion the idea that any existing DHW system needs to be thrown out and replaced with a new UVC based system. When heat pumps could only reach a flow temp of 50C or so, that was probably true. Now probably less so but the industry is a bit stuck in a rut IMHO (others may disagree).