That item about the hot water heater can be resolved by reading the tag/web site regarding the usage, from the manufacturer.
It seems that the hot faucet alone is the mixer! If you turn it on a bit, you get hot water, if you increase the flow, that is open the faucet more, you increase the amount of cold water passing through the heater, thereby making the water cooler. ( less hot) . Try it. No cold water faucet needed.