The manager obviously misspoke. He meant NYE. Maybe he was assuming you wouldn't be willing to move to a non-suite NYE if he upgraded you the other days. You could have easily clarified with him by asking how much it would cost to upgrade to a suite on any day other than NYE. I assume he would have said something around 2k baht per night.
Yes, I saw in your Flicker feed that last year (12/31/20) you were not in a suite NYE.
Hilton is not Hyatt and not SPG/Marriott and has been running behind both those chains for 15+ years. Trying to play catch up a few years ago Hilton said Diamonds "can" be upgraded to a suite. It's not a guaranteed benefit like it was with SPG, then Hyatt, and then Marriott.