I do product testing with Kinetic and have ridden a fair amount on the Kinetic InRide (name of the sensor that estimates power and broadcasts it) so let me help out here. The in-game calibration from Zwift isn’t setup to do this specific calibration. There are some confusing aspects of this for end users and manufacturers alike.
The advice to calibrate in the Kinetic Fit App is spot on though. If you warm up for 10 minutes with tire pressure to 100 psi you get good results. The calibration will guide you if you need to increase or decrease roller tension. In my tests the power values are frequently in the 5% or better match with a direct force power meter and quite consistent. In fact, if you leave the bike on the trainer you can just keep your tire pressure pumped up to 100 psi every few days and calibration becomes an infrequent matter only required if you move the bike on/off.
I hope this helps and it should be easier than the instructions to use it as a classic trainer. Happy to answer any other questions.