Dear Toolong, you where not slow. I was! I just answered you yesterday ????
Yes, this is very interesting and all this happen the last 3 weeks. So it is not old information. Chaeng Wattana was on Monday as I got the 1 year extension so that was a perfect opportunity to raise this question with them in the main Immigration of Thailand. But it was not known by them. Talked with the correct people there. Not the extension officer.
I would like to hear if others have any knowledge about this as well. I have also heard about people having this restriction (in Thai) on the backside of the license for many years already and not know about it as it is in Thai and they have driven all around in Thailand and also been stopped by the police several times with no problem ????. At least it is a good idea to ask at DLT next time about this and then choose passport or ID card. The person at the amphure called them and asked about this as it was news for him and then DLT just hanged up the phone! The man at the am amphure called 2 times with the same result. It was surrealistic!
It is worth to mention that this maybe can affect the insurance if having an accident. So better check with them as well.
I would be happy to hear if you find more information about this. I personally have decided to bring the international driving license with me all the time and after 2 years I will renew it with the passport for the 5 years driving license.