The Supreme Court didn't exactly say that. The court said it did not recognise Thammanat's conviction because the verdict was delivered in Australia, a foreign country.
"The verdict of any state only has effect in that state," the court said.
That offender was not indicted, nor convicted, in Thailand, Shinawatra has been indicted here, and his case will be heard in Thailand, there's a big difference. The Thai court's verdict is what will count.