Can't agree with that statement, simple boiling water in a kettle will not kill all bacteria and can actually promote the germination of cryptosporidium oocysts which are very resistant to heat and chemicals and are thought to cause most of the cases of stomach bugs in the UK. If you think about it a kettle boils for what 30 seconds at most and is not sufficient to kill many bacteria.
Only way to ensure killing most of the bacteria is to boil in a pressure cooker, cool and then re-boil in the pressure cooker. This allows the crytosporidium to bloom into it's vegatative form which is much more suceptible to heat.
It depends how the hot water tank is fed as to what it is likely to contain. If you have a gravity fed tank with a cold water tank in the loft then you are likley to have a higher chance of the water containing harmful bacteria due to the cold tank containing standing water unprotected due to a lack of sufficient amounts of chlorine. If you have a mains fed tank then the likelyhood of organic contamination is extremely low and comparable to the mains fed water taps. In both conditions you can have slightly higher amounts of copper due to the heating process, but again it is very unlikely to be to a level where you would be adversely affected with long term use as naturally there is copper in the water we drink.But a hot water cylinder works by having HOT (not boiling) water circulating through the inner copper pipes, to heat the water in the tank, so it never gets to boiling... and all sorts of weird stuff accumulates, some of which is kinda... jelly like
it is rank tbh. And every house has it.. and it's why you should NEVER drink hot tap water.
In fact if you are a fan of organic foods it is quite likely that you consume more copper from your food as Copper Sulphate solutions are considered to be an organic pesticide and are still legally and widely used in the organic grape growing business afaik.
With regards to the limescale then as a constituent part of water it is actually a requirement for human body function to consume those constituent components. If you soften hard water then it is actually advisable not to drink the softened water as it can contain high levels of sodium, depends on the method used. So a softener can be used on the hot water system supply to reduce scaling and it would then be advisable not to consume it in larger quantities.