This was done in Canada as well when the period between doses was initially set at 16 weeks. The intent was to make best use of the limited supply of vaccines to get first shots into as many arms as possible. Though a single shot does not give maximum immunity, it does provide some protection and probably reduces the number of cases that become severe. In Canada the results have been good, though the supply eventually got a lot better and now most people have received two shots. The time between shots for those that remain has been reduced to six weeks. Many countries with successful vaccination campaigns have done something similar.
The main problem in Thailand is the abject failure to get an adequate supply of vaccines, not the perfectly reasonable policy of extending the period between doses.