Got this detailed response from eufy on amazon:
For the time being, according to Homekit access requirements, they support Full Two-way audio that can receive and transmit audio through the IP camera at the same time or will not support Two-way audio. However, for IndoorCam, the two-way audio feature cannot receive and transmit audio at the same time, you need to press the “Hold and Speak” button on the live feed continually when you talk to the camera, and then you won’t hear the third party talking into the cam. At the current stage, we are not able to communicate with the one in front of the camera at the same time. Therefore, it is not supported by the Home APP at the moment.