Call TRD helpline #1161 for clarification.
TINs are for humans without Thai identification numbers, or companies and juristic persons. Your pink ID does have a Thai identification number, one which you use for tax filing.
The TRD person at Sathorn Rd is misinformed.
What's the need for an "expat desk", besides offering (hopefully!) English language support? Many of us have visited TRD offices and filed in person just like ordinary Thai citizens.
My TIN is different to my Pink ID/YTB number, even though I presented my Pink ID card and YTB wen I applied. I even attached copies to my application form.
My Pink ID number starts with a 6 but my TIN starts with a 0 (Zero)
Here is a discussion about TIN numbers NOT always being the same as the Pink ID numbers .