The 787 APU has two starter/generators each. One powered by the APU battery, one by "something else". If ships power is available, the starter that is powered by ships power starts the APU. Taxiing in the APU battery should not have been used for APU start.
I also read the 45 amps from the report. 45 amps wouldn't come close to starting a 787 APU. They presented the 45 amp flow as a fact, but I don't remember them guessing why.