Be assured, it is not the farang.

This will help: Section 71 Page 11 If, when entering a junction, there are other vehicles, the driver must let such vehicles go through first. If two vehicles enter a junction from different directions at the same time, the vehicle on the left side has a right of way, except when theres a designation of principal roadway in which case the vehicle on the principal roadway has right of way. http://thailaws.com/law/t_laws/tlaw0140_5.pdf

OK, that implies giving way to the right, which is congruent with using a roundabout. I thought giving way to the left was nonsensicNo

NO...It does not imply...it requires giving way to the vehicle on the left unless the roadway you are on is designated.