Based solely on watching how Tesla seems to process a lot of their big releases in waves, and then pause while lots and lots of cars get brought up to the latest version, it wouldn't surprise me if they have features that depend on software components, or firmware in the car being updated to specific versions before the next version comes out.
When track mode was released I think it was in a specific version of the car software that only went to a subset of model 3. Then those same cars were updated to later versions that were the same as other model 3s but their track mode option stayed available. It's possible that the software is identical and some cars just have a flag set to enable the option. It's also possible that there were some firmware updates to parts of the car that were only made as part of that special versions installer/update script, and the track mode UI option is just based on if you have the firmware version required in some deep part of the traction control system or whatever.
Effectively they have two ways of changing our cars, one being the version of the software that it's running, and the second being the changes they make during the update process itself. I think Tesla is likely to make sure that every piece of software and firmware controlled piece of tech in the car is capable of being updated to a new version OTA. There are probably dozens of little software driven components in every Tesla, so the path of least resistance for something like Track Mode where it touches a small subset of the total cars might include using the update process to change some of the lower level components of the full tech suite rather than the software that runs the more user facing aspects of the car.
This would mean that the only way to enable track mode on a car is through an update that includes the track mode "installer". They would probably want to make sure that they had all those firmware components rebuilt/updated to match the now current software, and make sure that everyone who is getting that version is caught up to one (or at least the smallest possible number) known good path for the upgrade because you don't want to end up with an incompatible state or a failed upgrade where half the car accepts the new version but whatever bit of micro-controller is running the torque vectoring or braking rejects an older version, or doesn't recognize the "turn on/off track mode" command.
That would explain the idea that they want to wait for people to get updated to a specific point, so that they don't have to test their special upgrade process on dozens or hundreds of different car software configurations.