There is no indication that Tesla didnt do their own testing. In fact they did as they had their own stopping distance, which they responded with to the CR report.
Since this can apparently be fixed by an OTA update it seems more likely that a bug happened/or some such during continued work on the car’s software.
Or the period of CRs testing (one mile cool down) was different from Tesla's and exposed an issue with the ABS adaptation algorithm. (such as multiple lock up events in X amount of time cause it to reduce braking force, like an ice mode)