No, that is not what I am saying.
I am sure the locally manufactured AZ vaccines are the same quality as AZ manufactured anywhere else, and the imported Pfizer/Moderna/Sinovac are the same as where they were manufactured.
I am saying that Thailand is the land of scams and forgeries, so how easy it would be to get a forged vaccine certificate??
There is also close to zero law enforecement in Thailand, so no-one is going to be stopping these forgeries being created and sold.
That is why the UK is not (yet) accepting vaccines ADMINISTERED in Thailand as valid.