As a child in the 40s/50s we didnt have running hot water we used to have to light the coke boiler to heat the water for a bath and boil a kettle for a wash or shave.so that wasnt relevant. Mind you dad passed away 35 years ago so times and attitudes have changed somewhat but I still wouldnt fill the kettle from the hot tap, old habits etc.
I also wouldn't fill a kettle from the hot tap, but I was really just pointing out that hot water cylinders don't contain stagnant water.
Well unless the cylinder was in a empty property and unused for many years of course.