Air gap means there’s no physical connection. Like literal air between one wire and the other. Air gap should never be thought of something like a “logical air gap” because so such thing exists. Either the cable is physically plugged in or it isn’t.
Pretty sure that the common definition (also the one from Wikipedia [0]) is that there's no connection to a network (including wireless ones; the medium is not an important part in the concept).
The article appears to say that a database/DBMS in the ideal world is not accessible over network at all. That is, apparently only accessible by users who have physical access to the machine it is on.
