I've had a look at this and it's still less than 100g/km to qualify for zero road tax. I know Boris is cutting the threshold to be exempt for the congestion charge due to too many cars qualifying and it wouldn't surprise me if a similar thing happenned with road tax at some point.

There is a lower rate for the first year of ownership for any car in Band D or below which means they pay no road tax for the first year. Band D is 130g/km and below so maybe he misread it as 128 rather than 138?