Wrong.
You better read Thai law.
There's no mention in the Thai penal code that your statement needs to be false.
Even if what you are saying is true, if you publicly impair someone's reputation, it's still defamation.
They main element of this law is that you: public impaired someone's reputation. That's it. Nothing about falsehood.
Private conversations are okay. You can say what you want -- even lies -- but not publicly -- even if what you are saying is true, it is still illegal to damage someone's reputation in Thailand.
https://library.siam-legal.com/thai-law/criminal-code-defamation-sections-326-333/
Section 326. Defamation
"Whoever, imputes anything to the other person before a third person in a manner likely to impair the reputation of such other person or to expose such other person to be hated or scorned, is said to commit defamation, and shall be punished with imprisonment not exceeding one year or fined not exceeding twenty thousand Baht, or both."