There were, as you've found, no British colonial regiments taking part in Operation Dynamo. However, there were six companies of mule drivers from Britain's colonies within the Dunkirk pocket - four from the Indian Army's Service Corps and two from Cyprus. These are not typically included on orders of battle, as they were not combat units. Their main role was to carry supplies forward over rough terrain. The order of battle given in the official British history of the BEF, found here, includes no units from the RASC, let alone the RIASC. It was also possible for individuals from Britain's colonies to serve within British units - as long as they'd travelled to the UK and joined up there, then they could fight within those units.

In addition to these Indian units, there were elements of ten colonial regiments from French North Africa in the pocket. The presence of these units would not be revealed by looking at OOBs for the British Expeditionary Force, because they were not part of said unit (being French, not British).

For more information on this topic, see here, here, and here.

You should note that the OP in that r/badhistory thread did not claim that any imperial or colonial soldiers were at Dunkirk in the BEF beyond the Indian support personnel, though they did comment on colonial forces in the context of the wider war.

The number of Colonial or Imperial troops in the BEF and at Dunkirk isn't really a matter of debate in that thread; French colonial forces are a matter of conjecture therein.