The main Wikipedia article on the Tito–Stalin split is not quite as definitive about the alleged invasion plans, saying simply that:
Between 1948 and 1952, the Soviet Union encouraged its allies to rebuild their military forces—especially Hungary, which was to be the leading force in a possible war against Yugoslavia.
The 2005 paper War on Tito's Yugoslavia? The Hungarian Army in Early Cold War Soviet Strategy appears to cast doubt on the suggestion that the Soviet-bloc actually planned to invade Yugoslavia in 1949. It notes that:
"Béla Király, Commander-in-chief of the Infantry of the Hungarian army in 1949-1950, claims that Stalin had made a decision to resolve the conflict by military force"
"The attack had allegedly been planned from 1949 onwards, and the huge military build-up, exercises and the final war game on the basis of the “actual plan of attack” were concluded by the spring of 1951."
The reason that the plans were shelved was that, according to Király,
"... after the forceful answer of the United States to the North Korean attack on South Korea, the Soviet leadership thought better of their plan and quietly abandoned it after 1951."
Indeed, on 14 November 1951, President Harry S. Truman did ask Congress to approve military and economic aid to Yugoslavia.
However, that paper actually concludes, based primarily on declassified documents from both sides of the Iron Curtain, that
"the Hungarian military was not preparing for an invasion on Yugoslavia as Király claimed."
Although it does so with the important caveat that:
"... the archives responsible for defence policy in Moscow have partly or completely denied researchers access to their holdings for the period under discussion"