Be interesting to see how they reply to your follow up.
Out of interest, I looked at a similar AWG, the Owon DGE2070. It appears that range of generators also behave the same as the UTG962. Here's what their manual says about AM modulation...
"In 0% modulation, the output amplitude is half of the specified value. In 100% modulation, the output amplitude is equal to the specified value."
Owon have got it wrong too IMO.