Utter garbage, a total fallacy.
The Southern provinces bordering Malaysia are hugely dependent on their neighbours for tourism, why would tourists fly there when it's more convenient and cheaper to cross by land. Same applies to other neighbouring crossing points, after all if it's good enough for Westerners travelling around SE Asia why wouldn't Asians from those countries do the same coming to Thailand for a holiday.
Yeah let's count every Cambodian and Burmese worker coming into Thailand and earning a pittance as a tourist so we can show a huge increase in our tourism revenue. Seriously ?
Aside from the fact the length of stay figures by those countries, which they publish, don't support your theory, they would be shooting themselves in the foot by lowering overall average spend per tourist - a figure they want to show increasing.