I dont think the article implies US pre-cleared services from the 300 gates. I think it implies that other wide-body services, such as the EI sun-flights, YYZ, EK/EY or non-pre-cleared services could depart from the 300s, rather than pre-cleared US services.
If push really came to shove passengers could be pre-cleared in the current CBP area and bussed to aircraft on remote stands, perhaps?