This is true, but the economic growth of the region during and after the 1970s was also very important. Pushing a regional trade agenda, and the importance of the SE Asian countries, propelled ASEAN. But when ASEAN was formed it was designed to be an economic and cultural forum.

ASEAN countries were wary of Vietnam (the Socialist Republic of Vietnam), in particular Thailand, because it bordered Cambodia. Throughout the Indochina wars, Vietnam had dominated the revolutionary forces in Cambodia and Laos. After 1975, the Pathet Lao came to power in Laos but were so subordinate to Vietnam that it was essentially a satellite state of Hanoi. Then in 1979, Vietnam invades Cambodia and drives what is left of the Khmer Rouge into Thailand. The countries of ASEAN were wary of a belligerent Vietnam that was growing and willing to challenge ASEAN and its non-communist member states.

Vietnam had already been making provocative statements about supporting regional revolutionary movements and had tried to circumvent ASEAN through bilateral treaties with SE Asian nations. Vietnam also critiqued ASEAN as a tool of American imperialism and proposed the creation of new regional body that would supercede ASEAN. In mid 1978 it proposed the Zone of Peace, Independence and Genuine Neutrality.

In this context the invasion of Cambodia becomes a point where ASEAN asserts itself as a more political organization against the political and military threat from Vietnam. And now outside powers were consulting ASEAN, seeing it as a the best counterweight to a Vietnam controlled Indochina landmass, and owing to Vietnam's siding with the USSR in the Sino-Soviet split, also an Indochina tied to the Soviet Union. ASEAN worked to deny foreign aid to Vietnam and the regimes in Laos and Cambodia, and publicly lobbied for the withdraw of Vietnam from Cambodia. ASEAN played an important role in defeating a UN vote to withdraw the Cambodian UN seat from Democratic Kampuchea (the Khmer Rouge) and hand it to their Cambodian occupation government. Because the US and China were allied at this point in opposition to Vietnam, they also lent support to ASEAN and encouraged its larger role. ASEAN even hosted an international conference on the issue in the early 1980s and continued over the next decade to lead talks for a resolution. To keep the various nations of ASEAN on board with this program, power had to be distributed among them, which also helped strengthen it in the '80s.

A final note, it's ironic that now it China's threat to the region that is pushing ASEAN to become a more political body and take a larger role in regional affairs.

Shuan Narine, Explaining ASEAN: Regionalism in Southeast Asia. David Elliot, Changing Worlds: Vietnam's Transition from Cold War to Globalization.