All that really matters is that the measurements are consistent. What is 250W, its just a number. What is 30km/h - just a number.
Think of the numbers coming off the trainer as being that - just a number. If you need to compare it to your road bike, you need a conversion functon. "Actual speed km/h = fx(Trainer 30km/h`)", and work out though measurement and experimentation what algorithm to use for fx(). IN the real world we have two systems of numbers and a simple fx() and no one bats an eye.
If you are doing it for training, all you need to know is your performance and how it is changing, for this, you just need numbers, and as long as the trainers numbers are consistent no need to worry about how they relate to the real world.
Real world has things like physiological effects of wind that actually cools the body and phycological effects such as levels of interest and boredom or fear of the large metal objects hurling towards your at 100km/h that impact different riders differently. Add differences in road surfaces and rider position, and there is no way a trainer can provide accurate numbers for everyone. Best they can do is provide an approximation that is sort of accurate for some people some of the time.