Unlike the other manufacturers, Tesla has a large number of vehicles on the road capturing data and that can be used for testing out new software while running in shadow mode in parallel with a human driver or previous version of the AP/FSD software.
It's very possible that Tesla has been testing Tesla Vision for quite a while - and based on the results of those tests - in real world conditions, they concluded they do not need to have radar.
The Tesla test vehicles with lidar are probably being used to do additional validation of Tesla Vision - comparing the object recognition (type of object, location, relative speed and direction, size, ...) of TV vs. lidar and possibly also against what radar is reporting.
They may even be recording the data streams in the test vehicles - and when there are area where Tesla Vision made mistakes, they can revise the software and re-run the scenario until they get it right.
Since they already have been manufacturing vehicles with radar - and removing the radar isn't going to be a huge cost savings, it seems likely they are doing this only after Tesla has done enough testing to demonstrate the radar data isn't needed.
This is a huge change for Tesla - and it seems unlikely they would make this change unless they were confident this was the right direction.
Though like any change Tesla makes, it could be a "two steps forward, one step backward" path, where it will take them a couple of releases to get all of the kinks worked out when deployed to the entire fleet.