If the speaker were a BIV speaking vat-English, the speaker would not even understand (T) in its English sense, let alone have warrant to believe it in that sense

Another way to see that the argument based on the English sense of (T) would be question-begging is to ask whether a speaker would have warrant to believe (T) in its English sense, if the speaker did not already have warrant to believe that she was a non-BIV speaking English. Note that if the speaker were a BIV speaking vat-English, the speaker would not even understand (T) in its English sense, let alone have warrant to believe it in that sense. So it surely seems that the speaker could not hav