From your post it seems you haven't travelled outside Thailand for a long time, so your TM6 is still valid from the last time you arrived, which is why they didn't discard it.
For people coming now (since 1st July I believe) there is no TM6 requirement, and the IO can refer to the entry stamp date in the passport if they want/need to check.
I know this from first hand experience.
TIP for all:::::::As many people have found out, the Airport IO requires to see your boarding pass (in lieu of the TM6) upon entry.
My local IO also suggested keeping your latest boarding pass, as proof of entry date (if ever required). Sounds a sensible idea.