I can’t seem to find any other sources that claim this and the number seems impossibly high. Where did the film get this information from? What’s the basis for this claim?
Yes, this is correct, although it's worth pointing out that the Chinese estimate of 250,000 includes people who died from all Japanese actions, which involved burning cities, destroying food crops, looting, indiscriminate killings, and biological warfare using paratyphoid and anthrax. I wrote about this more here.