Note that the FAQ relates to regulation 599i, and not the latest regulation 599f. But I can't find any redefinition on what vehilces constitute private space within the full text of 599f.
Is private vehicle a public place and whether the driver and passenger therein are subject to the mandatory mask-wearing requirement?
Generally speaking, private vehicles for personal use, if they are not available for hire by the general public and a person may not gain access to the vehicle for the time being as a member of the public, are not regarded as public place where wearing masks is required under the Regulation. In any case, drivers and passengers should wear masks as far as possible for prevention of the spread of disease.