After doing some research, it turns out that Michel Thomas is correct, but only in some cases.
1.) The past subjunctive can only be used in a clause by itself in 3 cases, none which translate as might:
- quisiera lit I wanted - (I / you / he) would like...
- pudiera lit I could / I was able to - Could (I / you / he)...
- debiera lit I would have to / should - (I / you / he) should...
2.) One use of the subjunctive mood in English and in Spanish is to show doubt or uncertainty. The word may shows doubt in the subjunctive present, and the past tense of may is might, hence it is the past subjunctive / imperfect subjunctive tense.
- no pienso que venga - I don't think that he MAY come
- no pensé que que viniera - I didn't think that he MIGHT come
- no pensé que hubiera venido - I didn't think that he MIGHT HAVE come
- no pensé que hubiera estado venido - I didn't think that he MIGHT HAVE BEEN coming
However, we would most likely say:
- that he is coming / that he is going to come / that he will come
- that he was coming / that he was going to come / that he would come
- that he had come
- that he had been coming
So, in reality, he really just needed to clarify that it cannot be translated as might by itself, and not in all cases.