> Also, what does this have to do with water heater use or fixtures?

It doesn’t particularly matter if small amounts of lead are used in hot water lines since you don’t (or shouldn’t) drink water from the hot water heater. Ideally your drinking water comes from a faucet that has separate hot/cold taps, if it doesn’t, then you should turn the faucet all the way to ‘cold’ when you’re filling a glass with water to drink.