The battery has 2 cells and is as big as advertised.
Scientific explanation:
Battery size (amount of energy) is measured in Wh 19.24Wh=7.4Volt*2,600Ah.
although the marketing unit is mAh, this is not logical, because mAh says nothing about a battery when the voltage is not given.
However, normally, 3.7V is the default value used.
The battery capacity is 19.2Wh. This is the 3.7V equivalent of 5200mAh.
The battery does have two 2600mAh 3.7V cells, they’re just connected in series, which doubles the voltage, instead of mAh.
Those two exact same cells in parallel would result in
3.7V *5200mAh = the same 19.24Wh