Getting good info from commercial websites like thaiembassy.com is very iffy. Even the legit sites for the many Thai embassies and consulates around the world contain details that are vague, conflicting, or sometimes just wrong.

To my knowledge, the 21 countries on the VoA list all get only 15 days (for a fee), with no extensions. They can, however, pay 1,900 baht to request an extension, get denied, and receive a 7 day grace period added to their permission to stay.

That's a rough deal compared to those countries on the visa exempt (aka visa waiver) list. They get in for free, get 30 days (or in five cases 90 days), and can extend for 30 days more.