on the actual Bangkok Bank letter at the bottom it states that “I warrant that I am the owner of the deposit account and I have received the certificate of deposit on the date of the request”. Sounds to me they issue the certificate on the day of the request and the bank then freezes the funds going forward.
like you said not an Immigration requirement but a banking one. Better for the bank to have one common banking process to communicate and train than several processes dependent on visa type.
I think you are reading it wrong. To me it reads that the bank will freeze (pledge) your funds of 400k/800k in your account and issue you the letter for immigration. After 4 months they will release the funds back to you. I don’t think it reads as prior but as frozen going forwards.