The comment was referring to "gas fired bath water heaters", which appears to be a specific type of heater fitted in bathrooms for heating the bath water…hence the potential for increased CO risk in the bathroom.

My hot-water + heating system is gas-fired, but the boiler is in a utility closet vented to the outside.

We have central heating but the boiler runs with a heat pump driven by electricity.

You are of course right in pointing out that central heating often goes along with central water heating and that heating is often based on fossil fuels such as gas.