One Australian Airforce page offers this nutshell version of why the Soviets agreed to air corridors but shut down land access to West Berlin in the late 1940s.
Berlin was in the centre of the Soviet Zone of occupation and 3 corridors were established to link the Western Zone with Berlin. The London Agreement of 1945 designated in writing Western access rights by air. However, agreement on other forms of transport access were verbal and not formally ratified. Accordingly, the USSR regarded a verbal agreement as not binding and used that to their own advantage.
I'm not entirely sure what the the "London Agreement" bit refers to though, as it's best known for establishing the Nuremberg Tribunal charter, but perhaps it had other (more obscure) issues in its remit.
So, was there a broader "London Agreement" that covered such issues like air access, or is the Australian page simply confused on the matter? The latter form of the corridor agreements that the US saw as binding was not agreed until Dec 1945, with minor changes made in 1946. But that does not preclude that the Soviets may have previously agreed--in principle and in writing--to provide such air access, as the Australian page suggests. But I'm most doubtful this was in the venue of a "London Agreement".
N.B. The London accords of 1947-1948 may be what it's actually meant here... (I don't know if such an assurance was given at the latter conferences either.) Furthermore, the rest of the claim, that the USSR simply disallowed land traffic on the basis that it had only been verbally agreed to, can't immediately be double-checked from the other source I consulted. According to another (more detailed) account, the Soviets simply and literally claimed there were (suddenly) "technical problems" with these...
Actually, the London Protocol of 1944 seems the more plausible thing being referred to, as it established the future zones of control, including the partition of Berlin. However, I'm unable to verify if anything was agreed in writing regarding air access back then, to substantiate the gist of the story in that quote. (Also Wikipedia is misleading as to when Berlin's partition proper was agreed. That only [p. 596] happened at the September 1944 Quebec Conference, when the reluctant US administration agreed to a partition of Berlin. Until then, the protocol only specified that Berlin was to be jointly occupied by the allies, but there was no partition defined. In general, throughout most of 1944, the Roosevelt administration opposed "detailed plans for a country which we do not yet occupy"-- Roosevelt's own words.)