In ESXi you have two types of port groups.
- VMkernel port groups - used by ESXi itself (e.g. Management Network, vMotion, etc.)
- Virtual Machine port groups - used for virtual machine network traffic
These port groups can be created on vSwitches, which then usually have uplinks (vmnic) to physical switches.
What the statement means, is that all port groups are created on a single vSwitch, rather than distributing the port groups on multiple vSwitches. The mentioned requirement is actually the default configuration that you see after setting up an ESXi host.
André