This is spot on. I would not expect any cheap BKK fares in January to be available last minute/later on. If we were talking about April (after Songkran) to June it would be a different matter, but not January/high season (flights to/from Thailand will be full).
I would buy the ticket now - waiting will most likely mean higher fares rather than lower/promo fares becoming available. I can almost guarantee you that you will not be able to fly on a "error fare" (neither to BKK nor elsewhere).