A public cloud is an association of EDR-capable ports that allow devices on the EDR IB fabric to communicate off of the fabric. To connect devices on the IB fabric to the rest of the network, the public cloud requires a gateway port, which is also called an uplink. With an uplink, traffic can move freely between a company's IB fabric and Ethernet network, including possibly an external gateway to the public internet. You cannot create a public cloud without an uplink. If you do so, you prohibit north-south traffic and enable only east-west traffic, which is effectively a standard PVI cloud.
The process of configuring VNICs for EDR includes connecting to a public cloud, not a PVI cloud. The presence or absence of a gateway port determines whether the traffic remains on the EDR fabric or can move through the Ethernet network too.