Apple AirPods watt hour meaning
I am wondering if I can say : "hmm, if the Wh rating of AirPods Charging Case is 1.52Wh, then a 5V adaptor would supply 1.52 / 5 ~= 300mAh 


Your calculations are correct if we ignore losses. 300 mAh at 5 V = 1.5 Wh and this is the energy removed during discharge so that has to be put back in during charging. The problem is that there will also be some heat generated in the battery so this has to be included in the calculations. 10% power loss is probably about right.


  ... and make it full in a little over an hour since the charging case has the capacity of 398mAh."



I'm not sure what a "charging case" is.
You have assumed in your calculation that the battery will charge at a one-hour rate. This may not be the case and it is not specified in the table you posted. It could be a 2.5 h rate or any other value chosen by the manufacturer. The charge rate will be determined by the battery management system (BMS) rather than by the recharge power supply.