Op-Amp / In-Amp (INA826) as voltage divider
The one thing I don't understand though is that I thought the way to
do it would be just to do a voltage divider followed by a unity op-amp
circuit but that is not the case

The input voltage range on your INA826 schematic goes from -10 volts to +10 volts and so you need an amplifier that has a negative supply voltage as well as a positive supply voltage to handle the negative range.
The 2.5 volt reference voltage (REF3225) on the REF pin then centralizes a 0 volt input voltage to produce +2.5 volts on the outputs hence, the output maps an input of -10 volts to an output of +0.2 volts AND, an input of +10 volts to an output of +4.8 volts.

we'd like to scale the op-amp output from 0-10V to 0-5V.

As it current stands, the output range is +0.2 volts to +4.8 volts and would be acceptable for driving a 0 to 5 volt input range device.

The only problem is that we only have access to a asymetrical voltage
supply (+5V)

Well, that's a problem if you need a negative voltage input range to map properly to the output.