It appears that AMCI testing
It appears that AMCI testing knew who was paying for the test. In other words, this was not blind testing, or they would have stated such, so that alone makes it invalid. In a valid test, the testers would have no idea who paid for the test, or whose product they were testing. Also they never stated that the cars were tested with OEM equipment, so they could have been modified.
Nothing is said about tires, so perhaps snow tires were put on the RAV4 and summer tires on the others. Nothing was said about tire pressures either, so perhaps the ones on the RAV4 were optimized and the others deoptimized.
It makes sense for manufacturers to supply cars with 3 season tires that are quiet, get good gas mileage, and perform decently on warm pavement. Someone with specific needs will buy tires specific to those needs. All my cars have winter tires mounted on their own wheels, so I personally prefer buying a car with tires biased toward warm weather driving, since they will not be on the car during the winter. I would not like the RAV 4 if the tires that gave decent winter performance were noisy and poor handling during the summer.
As to no claim being retracted due to a legal challenge. That is meaningless. Any smart company would retract a claim under threat of a legal challenge, before that legal challenge is ever made.
As to a legal challenge, suppose Subaru threatened one. AMCI might report that Toyota supplied them with several vehicles, prescribed the method of testing, and then AMCI tested them. They might say that they had nothing to do with choosing tires, tire pressures, or even verifying the condition of the vehicles prior to testing. That was all Toyota's decision, based on their contract with Toyota. So, if Toyota rigged the vehicles, that was not AMCI's problem. They were paid to test and that is all they did.
I hope that the other companies get together and research this testing and debunk it. I hope Toyota takes it on the chin, same as Volvo did years ago when it was found they reinforced the roof on the bottom car in the commercial where they had 4 cars placed one one on top of another.