The case of the 12 women, all in their 20s or 30s, presents a thorny problem for President Moon Jae-in of South Korea, who held a summit meeting with North Korea’s leader, Kim Jong-un, last month to discuss improving ties. Mr. Kim’s government has long demanded the women’s repatriation. But the South has always denied abducting North Koreans, saying that about 30,000 North Koreans who have arrived in the South since the 1990s were defectors.

The women were among tens of thousands of North Koreans working abroad who funnel badly needed foreign currency to their government. The North selects loyal and relatively affluent citizens to send abroad as workers, and the defection of the 12 women was a major coup for Park Geun-hye, who was South Korea’s president at the time and whose conservative government cited it as a sign of disillusionment with Mr. Kim among North Korean elites.

But many details of the defection remained a mystery and fueled suspicion, including how the restaurant workers managed to plot their escape despite being trained to spy on one another for signs of disloyalty. They also arrived in South Korea only two days after they fled their restaurant in the Chinese city of Ningbo, in the eastern province of Zhejiang. Other defectors usually took months to complete the trip to South Korea, often trekking through the jungles of Southeast Asia with the help of human traffickers.

Ms. Park’s government also took the highly unusual step of announcing their defection the day after their arrival. But it kept their whereabouts secret and blocked human rights lawyers from meeting with them. It also denied the North’s claim that the waitresses’ manager had conspired with the South Korean spy agency to take them to the South after telling them that they were being relocated to a restaurant in Southeast Asia.

But Mr. Heo says that was exactly what happened. Like other North Korean workers abroad, the women were trained to obey their manager, who held their passports.

“I just told them that we were moving to a new place,” he told JTBC.

Mr. Heo said he decided to spy for the National Intelligence Service in 2014 after Kim Jong-un executed his own uncle, Jang Song-thaek, on sedition and corruption charges. Mr. Heo said he became disillusioned with Mr. Kim after five of his former classmates were executed in a purge of officials close to Mr. Jang.

Mr. Heo said he met a South Korean agent in a motel in China, signed a letter of allegiance, and had his picture taken with a South Korean flag as proof that he would not betray the agency.