Maybe. However, when I was there in January/February 2020, there were plenty of foreigners there, including Chinese, Koreans, etc... and as you mentioned there were already some cases in January, and borders have only been closed at the end of March. In Italy, the pandemic probably started with one identified case, same for NY, and it diffused quickly. Actually no one really knows why SEA has not experienced an Alpha variant pandemic. NZ and Australia, It's another story.
https://www.nytimes.com/2020/07/16/world/asia/coronavirus-thailand-photos.html
In case all merits should be attributed to the government's policy for the low first wave, which is doubtful, it remains that they did not manage well the next one.