Maybe they meant natural gas (actual gas) rather than the liquid fuel we call "gas" as short for "gasoline".

Biogas can be generated by fermenting sewage. All buses in Stockholm run on it, it burns extremely clean and is carbon-neutral since it utilizes carbon already in the present carbon cycle.

All yeast strains produce some amount of hydrogen sulfide during fermentation as a by-product of sulfate processing. [1]

You can barely call that biogas "burns extremely clean" with hydrogen sulfide in them, can you?

[1]: https://beerandbrewing.com/off-flavor-of-the-week-sulfur/

Sorry, bad choice words. The work is not done by a yeast so perhaps it’s not technically fermenting. The work is done by a type of bacteria called archea. As far as I know the only byproduct Upon burning biogas is CO2 and water vapor.

Low emissions.

The alternative IC engine is typically diesel and spews lots of carcinogenic particles and compounds.

You may have a point. It burns cleaner ( liquified natural gas ).