Those points could be used to connect breakout drivers, but as those channels are configured in firmware for TMC2660 drivers, it would require a firmware change to support Pololu-type drivers on those pins. Given that the firmware already supports connecting Pololu-type drivers to the expansion bus, and mapping of XYZ/E1/E2 motors to whatever drivers you want, it seems pointless to complicate the firmware by making the on-board driver type configurable.
This makes sense, also the onboard drivers are IMO far superior to most. No need to complicate things more.
Jeff