I have B&M e-Werks on both my Extrawheel trailer and one of my bikes. They don't preclude use of a taillight. It all depends on how the whole lighting/charging circuit is wired.
If it helps I have dyno-powered taillights on both e-Werk installations. They all play together nicely. The headlight is switchable and the taillight is slaved t run off the headlight, so it turns off and on with the headlight. On the trailer, I made my own USB DC power supply to rectify and limit the AC current supplied by the SON dynohub. I installed a microswitch on the rectifier/voltage limiter so I can turn off the taillight when I want to put all the hub's power into charging via the e-Werk. In practice it was probably overkill as the taillight (an LED B&M Toplight Line Plus) requires very little power even compared to my LED B&M Cyo headlight.
I can't see any charger-related reason why the taillight isn't connected.
Best,
Dan.