Basement/garden apartment rented to non-family members?

On a DOB web site I find:

Basements in one- and two-family homes can be lawfully occupied only if the following conditions are met:
(requirements for minimum room size, ceiling height, etc)
plus
“The basement is occupied by only members of the family or families occupying the dwelling.”
Is it really true that the owner of a 2-family building cannot rent out an otherwise legal basement apt to non-family members? I thought people did that all the time.

i think what they’re trying to say is that in a 2-family building the occupants of the basement must be members of one of the families occupying the building already. if the basement was occupied by another party in addition to the 2 families it would be an illegal conversion to 3-family. Does that make sense?