The blunt tools all exist. ไม่ใช่, ผิด and ไม่เห็นด้วย are real Thai and you will hear all three. They are also heavier in Thai than their French equivalents are in French, not because the words are stronger but because the surrounding norm is that flat contradiction is avoided.
So the frequency is what matters. Used at French rates, correct Thai reads as aggressive.
What people do instead
หรอก is the particle for gently taking something back. It says 'that is not quite it' rather than 'no', and it is the cheapest of the three moves here.
Conceding first and then adding is the second: agree with the part that is true, then supply the rest. ก็จริง does this in two syllables.
Downgrading yourself is the third, and it is the one that scales: ไม่แน่ใจ, I'm not sure, does the work of 'you are wrong' while putting the doubt on you rather than on them.
- ไม่ใช่หรอกmâj tɕʰâj rɔ̀k
no, that's not it
softened; the หรอก is doing all of the work
- ก็จริงก็ tɕiŋ
that's true, as far as it goes
- ไม่แน่ใจmâj nɛ̂ː.tɕaj
I'm not sure
the most useful disagreement in the language
An honest limit
This is not a rule that foreigners must be indirect, and there are situations, a wrong bill or a safety problem, where being plain is correct and expected.
It is that a flat no carries weight here that it does not carry in French, and if you spend it at French frequency nobody will tell you. That is the week 4 article again: the correction you would need in order to notice is exactly the correction nobody is going to give you.