Thank you for the answers. I thought the reason could only be that on exiting at the particular wing I was in at BKK they only had the E-Gates.
The only online government paper I can find stating the right of foreigners to use the Thai line at the moment is from the dept. of trade negotiations, page two row 3.
https://api.dtn.go.th/files/v3/5d776ff5ef41406e2a4ee3de/download