I don’t know what “fill it in as blank” means when related to your tax number.
About the bank’s attitude: They are legally bound to do this due to CRS requirements. Every bank I know lists in its T&C that you are required to provide truthful and complete answers to their information requests or they *may* close your account.
“no one knows” is the biggest bartalk story I keep hearing from people who don’t go beyond sticking their heads in the sand and simply deny that there might be a liability.
Tax liabilities are different for each individual here, so a five lined post will never bear enough information for a definitive answer. Giving indefinite answers is the only choice.
“Wir haben es nicht gewusst” will not protect anyone from legal obligations. The excuse has been attempted and tested before.
On top of that I see a huge language incompatibility here. Most Thai people have limited or no command of the English language and if the foreigner also has language problems (see post) then that is by itself a guarantee for misunderstanding.