nope not confused. I left Us on 29 May and applied on 27 May. It was denied since my flight did not arrive in BKK until 31 May. Your flight took half the time.
within 3 days of ARRIVAL date. I was so excited and tried to do it 3 days before I left US only to discover that due to the 24-hour long trip, that your advice is wrong.
My grandson is half-Thai. We had to start the passport application in our home country (USA). It’s much better than waiting at a later time. The U.S. embassy in BKK would not do the application. Maybe your country is different. You should check.