My wife's whole village is Khmer speaking, together with the surrounding villages near to Krasang. I have visited the local primary and secondary schools on many occasions. Effectively they are 100% second language Thai schools. All the children speak Khmer at home and learn Thai in school.
There is oodles of evidence that being brought up bi-lingually makes learning further languages that much easier. Bi-lingual children are behind their mono-lingual counterparts at age 5, in terms of total vocabulary, not surprising as they are learning two words for every object and concept. By the time they are 8 or 9, however, they forge ahead and far outstrip their mono-lingual colleagues.
It doesn't surprise me in the least that Isaan Khmer speakers, or for that matter Lao speakers, would reach higher levels of English than those brought up only with Thai.
My grandson (two years old) dropped something on the floor the other day and said "Whoopsie daisy". I wasn't even aware I had taught him that...apparently he only needed to hear it once or twice to assimilate it. I remain thankful he didn't come out with, " Oh, shit!" For another day perhaps!