I work at TestFlight. Short answer is: set it to YES.
Long answer:
@Kerni is correct. Before we started symbolicating server side, we needed that data to symbolicate on device.
So if you upload your dSYM to TestFlight, you can strip them. If you don’t want to upload your dSYM for some reason, you can leave them in and TestFlight can symbolicate on device.
Basically the README was not updated. Sorry @Manni! I will update it for the next release. (I’m going to remove that entire section).
Sorry about the confusion!