Let's leave aside their ignorance that burning hydrogen IS combustion.
What they are trying to say is that hydrogen/oxygen combustion is non-polluting, which is true. Its "product of combustion" is water vapor.
The greenies will be able to pat themselves on the back for driving a non-polluting car. This will be true, but only by moving the pollution to the power plant. Unless the electricity to make the hydrogen is produced via a non-combustion process.
BTW, does anybody know if this "hydrogen combustion releases only water vapor in the exhaust" meme is actually true in the field? The reason I bring this up is I work a lot with ozone, which it produced by an electrical corona discharge field.
Run 02 through the field and pure O3 (ozone) and leftover O2 comes out.
Run dirty and humid air through it and you get all kinds of crap in addition to the ozone: nitrogen dioxide and other nitrous oxides, nitric acid, etc. This is almost universally ignored by people who pontificate about O3 production.
Does something similar happen when you combust hydrogen with air instead of oxygen?
I have been following fuel cell technology and its more of a chemical reaction than true combustion. The hydrogen molecules will only bond with oxygen molecules.