Being in the industry these "sold out" units are sold. The developer may withhold a small percentage of the best units to maximise on their value later in the construction process. A lot are bought in multiple unit lots by investors/speculators that then flip them as the price/sqm rises during construction. Recently a very large sub-penthouse sold in 2022 for 150k/sqm was "flipped" to a Chinese buyer for 300k/sqm.
Many units that are sold off plan are also re-sold prior to completion due to circumstances changing for the original buyer including their financial inability to complete payments and transfer. This was a huge factor a decade or so ago when the Russians were forefront of the market. They would be on their 2 week holiday from the cold and pay a minimal deposit on 2,3,4,5 or more units and when they got back to reality and the cold and payed a few regular instalments that they realised they couldn't afford what they'd bought. The developer gets to keep the money they've already received and they get to re-sell at a higher price and the agent can sell again and get another commission.
In short, there is a huge turnover of units during the course of pre-construction and construction.
Last week was the launch of another 50 level residential high rise on Pattaya 3rd road. Construction is not scheduled to even commence until Q1 2026 yet a group of European investors secured in excess of 400,000,000 Baht on the first day. These will be flipped without doubt during construction.
There's your answer.