It is unclear which "it's" you are referring to. Group sex dealmaking or using group sex for bonding business partners as per the original article? I don't think that is normal in Thailand or any Asian society for that matter.
As for the history of prostitution in Thailand. That's a well covered subject. You're points are somewhat accurate but OP's point appears to be different than what you are addressing. I thought he/she was implying that the Vietnam war had overarching societal and economic consequences, one of which was the availability of vulnerable exploitable migrant and rural women in Thai cities.
https://www.endslaverynow.org/blog/articles/history-of-prost...
The location of Thailand plays a key role in the success of the sex trafficking industry. It is close to war-torn Myanmar, Laos and Cambodia. China and Vietnam are also nearby. Various waterways along with porous borders also facilitate trafficking.
If the point that was being made was that the Vietnam war had huge side effects, including enabling horrific sex trafficking and exploitation, then yes, I would agree.