Integrated Gigabit switch with 4 x 10/100 (Fast not Gig) ports
What's the use of a Gigabit switch that you can't use?
Someone has gleaned a chipset specification not intended for the end-user and which only makes sense for the router manufacturer. It means the manufacturer can use the integrated 10/100 ports, or take advantage of the option to add their own Gigabit ports without needing an extra switch. Since this router does the former, the internal specification is therefore indeed useless/meaningless to the end-user.
It does not refer to backplane capacity of the switch - these modern, small consumer switches are all usually capable of line speed on all ports simultaneously, so if it said 100 Mbps switch there would be no practical difference.