Do I need to declare my computer or smart device when arriving or departing Thailand?

It's not about logging out of your device, it's about declaring the device to customs when you leave Thailand so that when you come back with it, they don't charge you customs duty on it.

It's one of these things that has been there forever, and most other countries have similar arrangements, but they are not frequently applied or matter, because travellers are rarely checked at the airports anyway. But then there was an announcement and the media ran with it... it's more an issue with stuff through the post, if you send something out of Thailand for warranty service customs will charge it on the way back. Unless you declare it in advance, and there is a procedure for this.