It does not have 8 ADCs but 4.
Perhaps it's the way they're describing the differential inputs.
P26:
HMCAD1511 is a multi Mode high-speed, CMOS ADC, consisting of 8 ADC branches, configured in different channel modes, using interleaving to achieve high speed sampling. For all practical purposes, the device can be considered to contain 4 ADCs.
P13:
Quad channel mode interleaves 2 ADC branches, dual channel mode interleaves 4 ADC branches, while single channel mode interleave all 8 ADC branches.
Edit:
By my calculation you need 8 ADCs to do 1Gsa/s from a 1GHz clock, although I'm confused by the timing diagrams.
Datasheet states it clearly that it is not just 8 bit ADC internally, on DS page 23: "There will be no missing codes for gain settings lower than 32x (30dB), due to higher than 8 bit resolution internally.
Analog resolution might be 13 bit but the ADCs are 8.