Futher reply from TfL: “Unfortunately all Great [sic] Anglia travel cards are valid for zone -6 and only a select few stations up to zone 9. I t seems the stations you mentioned are not covered within this travel card which is why it was not accepted by the reader. This information can be found on the Great Anglia Website.” (I assume the “zone -6” is a typo and should be read as “zone 1-6”.)
So back to GA. I’ve suggested to GA that they liaise directly with TfL and let me know the result. I shan’t hold my breath…