I'm not sure what your knowledge level is, so I'll go back a few steps.
Oil gets thinner as it warms up, quite a bit through the temperature an engine will span from cold start up to normal operating temperature. A single grade oil that works well in a cold engine will be poor at operating temperature. If this single grade oil is optimised for normal operating conditions, it's going to be like treacle in a cold engine.
Multigrade oil overcomes this by changing viscosity characteristics as it heats up, resulting in a constant effective viscosity through the temperature range.
Oil works as a lubricant by creating a film between the two parts so that they don't rub on each other. Lubricity is a measure of how well the oil does this. Lubricity is a function of viscosity and the oil's chemistry. Thicker oils create a thicker film and improve separation between the parts (up to the limit of the tolerances between the parts).
Oil chemistry looks at improving the oil's ability to prevent contact and friction at a given film thickness (basically making it more slippery). This is a way to give low viscosity oils better lubricity. It's a combination of basic oil chemistry and additive packages.
Lower viscosity oils take less energy to pump around, reducing parasitic losses. They will also make it into all of the oil galleries faster from startup, so you improve cold running protection. Manufacturers like this because they can improve efficiency in the first instance, and protect the engine better from ignorant owners in the second.
An additional consequence of lower viscosity is that it allows tighter tolerances between parts which can improve performance.
So that 0W8 oil is going to be pretty thin all the way through the operating temperature range, and provided the chemistry is good enough, it should be more reliable and efficient.
A side note on test cycles: they are not a reliable prediction of how the vehicle will perform on the road due to the variation of real world conditions from the test. What they do give is a basis for comparison: a better WLTP figure will on average mean better fuel consumption/lower emissions on the road. So if Toyota's test with the 0W8 has improved its official test rating, then it will also have improved its real world performance by some amount. Even if the test is done in bad faith by the manufacturer, basic physics still dictates that improvement in the test figure will improve real world too.