You are correct that pico-examples/pico_w/bt/config/btstack_config.h does not defines ENABLE_LE_DATA_LENGTH_EXTENSION, which is needed for DLE. To enable DLE, you can add it to btstack_config.h yourself.
@peterharperuk could you add ENABLE_LE_DATA_LENGTH_EXTENSION to the default configuration in the pico-examples repo?
See https://github.com/raspberrypi/pico-sdk/issues/1342
You are correct that pico-examples/pico_w/bt/config/btstack_config.h does not defines ENABLE_LE_DATA_LENGTH_EXTENSION, which is needed for DLE. To enable DLE, you can add it to btstack_config.h yourself.
@peterharperuk could you add ENABLE_LE_DATA_LENGTH_EXTENSION to the default configuration in the pico-examples repo?