FWIW, I have flown BKK-CNX and BKK-LPT (Lamphang) many, many, many times on PG (Bangkok Airways). I always check-in on-line, and proceed through the domestic security line, where they do not ask for an ID.

Sometimes I purchase my own tickets on-line so no need for a passport in hand. Other times a third party purchases a ticket for me in my name. Again, on-line check-in, so no need for an ID.

That said, YMMV, and I do always have my passport with me when I travel.