Yes, Berlin was an island isolated inside East Germany. However, the Berlin airlift was mounted when the communists embargoes the city by land and we had to resupply everything by air. So I guess in other times there were land deliveries into West Berlin.
However you could not just drive across East Germany into West Berlin. East and West Germany were separated by an armed border and I believe by a wall in many places.
Then, how were those 'land deliveries' delivered? In fact, there was a highway from West Germany into West Berlin that passed through East Germany.
Only the Helmstedt Autobahn was available for American forces to travel over eastern GE to Berlin.