I think we are missing a step in the functionality of charging here. The switch has a battery of 4310 mah, which is about 16W/h of energy. A 2.4A charger would give 12W, which means that the Switch should charge in 80 minutes. Additionally, even when running, the system takes at most 6.4W of power (16(W/h)/2.5h), so the power bank mentioned should be more than suited (even when considering a slight decrease in efficiency). It would seem that the charging power of the Switch AC adapter takes into account a charging time of 3 hours when playing as well, otherwise I can't understand why it would take 3 hours at 13W of charging power. As a result, the power bank should be able to charge the switch when playing at just a slightly slower rate than the AC adapter, but at the very least it will charge it in this setup.